Provide default implementation of LinkFile, don't break the build
Provide default implementation of LinkFile, don't break the build

Summary: By providing default implementation of LinkFile, we don't break other implementations of Env.

Test Plan: none

Reviewers: rven, dhruba

Reviewed By: dhruba

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29355

kaschaeffer/rocksdb,SunguckLee/RocksDB,ryneli/rocksdb,alihalabyah/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,mbarbon/rocksdb,biddyweb/rocksdb,ryneli/rocksdb,luckywhu/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,amyvmiwei/rocksdb,lgscofield/rocksdb,amyvmiwei/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,OverlordQ/rocksdb,NickCis/rocksdb,jalexanderqed/rocksdb,temicai/rocksdb,RyanTech/rocksdb,sorphi/rocksdb,vashstorm/rocksdb,norton/rocksdb,skunkwerks/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,alihalabyah/rocksdb,tizzybec/rocksdb,Andymic/rocksdb,kaschaeffer/rocksdb,Applied-Duality/rocksdb,hobinyoon/rocksdb,JohnPJenkins/rocksdb,mbarbon/rocksdb,wenduo/rocksdb,fengshao0907/rocksdb,wenduo/rocksdb,biddyweb/rocksdb,Applied-Duality/rocksdb,SunguckLee/RocksDB,sorphi/rocksdb,anagav/rocksdb,skunkwerks/rocksdb,virtdb/rocksdb,wenduo/rocksdb,RyanTech/rocksdb,wlqGit/rocksdb,wskplho/rocksdb,Andymic/rocksdb,hobinyoon/rocksdb,caijieming-baidu/rocksdb,rDSN-Projects/rocksdb.replicated,makelivedotnet/rocksdb,JohnPJenkins/rocksdb,norton/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,amyvmiwei/rocksdb,wskplho/rocksdb,skunkwerks/rocksdb,virtdb/rocksdb,skunkwerks/rocksdb,flabby/rocksdb,kaschaeffer/rocksdb,kaschaeffer/rocksdb,bbiao/rocksdb,skunkwerks/rocksdb,IMCG/RcoksDB,vmx/rocksdb,zhangpng/rocksdb,geraldoandradee/rocksdb,Vaisman/rocksdb,mbarbon/rocksdb,JackLian/rocksdb,fengshao0907/rocksdb,Andymic/rocksdb,lgscofield/rocksdb,JohnPJenkins/rocksdb,sorphi/rocksdb,JoeWoo/rocksdb,norton/rocksdb,ylong/rocksdb,facebook/rocksdb,wenduo/rocksdb,flabby/rocksdb,Vaisman/rocksdb,sorphi/rocksdb,vashstorm/rocksdb,biddyweb/rocksdb,JoeWoo/rocksdb,IMCG/RcoksDB,wlqGit/rocksdb,amyvmiwei/rocksdb,virtdb/rocksdb,caijieming-baidu/rocksdb,hobinyoon/rocksdb,flabby/rocksdb,vashstorm/rocksdb,tizzybec/rocksdb,mbarbon/rocksdb,RyanTech/rocksdb,vmx/rocksdb,tizzybec/rocksdb,zhangpng/rocksdb,bbiao/rocksdb,sorphi/rocksdb,JoeWoo/rocksdb,temicai/rocksdb,siddhartharay007/rocksdb,geraldoandradee/rocksdb,OverlordQ/rocksdb,rDSN-Projects/rocksdb.replicated,makelivedotnet/rocksdb,luckywhu/rocksdb,rDSN-Projects/rocksdb.replicated,makelivedotnet/rocksdb,fengshao0907/rocksdb,dkorolev/rocksdb,caijieming-baidu/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,zhangpng/rocksdb,tsheasha/rocksdb,rDSN-Projects/rocksdb.replicated,ylong/rocksdb,ryneli/rocksdb,lgscofield/rocksdb,dkorolev/rocksdb,Applied-Duality/rocksdb,kaschaeffer/rocksdb,NickCis/rocksdb,geraldoandradee/rocksdb,tizzybec/rocksdb,vmx/rocksdb,ylong/rocksdb,tizzybec/rocksdb,RyanTech/rocksdb,OverlordQ/rocksdb,RyanTech/rocksdb,tsheasha/rocksdb,siddhartharay007/rocksdb,vmx/rocksdb,skunkwerks/rocksdb,makelivedotnet/rocksdb,temicai/rocksdb,Andymic/rocksdb,NickCis/rocksdb,ryneli/rocksdb,siddhartharay007/rocksdb,wenduo/rocksdb,ryneli/rocksdb,rDSN-Projects/rocksdb.replicated,wat-ze-hex/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,ylong/rocksdb,JohnPJenkins/rocksdb,wenduo/rocksdb,vmx/rocksdb,luckywhu/rocksdb,skunkwerks/rocksdb,virtdb/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,alihalabyah/rocksdb,kaschaeffer/rocksdb,jalexanderqed/rocksdb,anagav/rocksdb,siddhartharay007/rocksdb,alihalabyah/rocksdb,rDSN-Projects/rocksdb.replicated,anagav/rocksdb,SunguckLee/RocksDB,JackLian/rocksdb,JohnPJenkins/rocksdb,makelivedotnet/rocksdb,wat-ze-hex/rocksdb,biddyweb/rocksdb,SunguckLee/RocksDB,dkorolev/rocksdb,hobinyoon/rocksdb,anagav/rocksdb,IMCG/RcoksDB,temicai/rocksdb,luckywhu/rocksdb,anagav/rocksdb,JoeWoo/rocksdb,vashstorm/rocksdb,wskplho/rocksdb,siddhartharay007/rocksdb,virtdb/rocksdb,bbiao/rocksdb,siddhartharay007/rocksdb,zhangpng/rocksdb,biddyweb/rocksdb,tizzybec/rocksdb,lgscofield/rocksdb,temicai/rocksdb,dkorolev/rocksdb,wskplho/rocksdb,NickCis/rocksdb,biddyweb/rocksdb,JohnPJenkins/rocksdb,JackLian/rocksdb,tsheasha/rocksdb,JoeWoo/rocksdb,Vaisman/rocksdb,temicai/rocksdb,JohnPJenkins/rocksdb,Andymic/rocksdb,wlqGit/rocksdb,amyvmiwei/rocksdb,ryneli/rocksdb,Andymic/rocksdb,wskplho/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,sorphi/rocksdb,luckywhu/rocksdb,flabby/rocksdb,JackLian/rocksdb,bbiao/rocksdb,JackLian/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,JoeWoo/rocksdb,fengshao0907/rocksdb,geraldoandradee/rocksdb,wat-ze-hex/rocksdb,fengshao0907/rocksdb,vashstorm/rocksdb,hobinyoon/rocksdb,dkorolev/rocksdb,vashstorm/rocksdb,caijieming-baidu/rocksdb,facebook/rocksdb,NickCis/rocksdb,virtdb/rocksdb,caijieming-baidu/rocksdb,vmx/rocksdb,JackLian/rocksdb,ylong/rocksdb,Applied-Duality/rocksdb,flabby/rocksdb,JackLian/rocksdb,virtdb/rocksdb,wat-ze-hex/rocksdb,flabby/rocksdb,Vaisman/rocksdb,facebook/rocksdb,ryneli/rocksdb,zhangpng/rocksdb,ylong/rocksdb,Vaisman/rocksdb,Vaisman/rocksdb,geraldoandradee/rocksdb,SunguckLee/RocksDB,fengshao0907/rocksdb,wenduo/rocksdb,alihalabyah/rocksdb,dkorolev/rocksdb,wat-ze-hex/rocksdb,tizzybec/rocksdb,alihalabyah/rocksdb,caijieming-baidu/rocksdb,RyanTech/rocksdb,NickCis/rocksdb,vashstorm/rocksdb,IMCG/RcoksDB,zhangpng/rocksdb,luckywhu/rocksdb,Applied-Duality/rocksdb,dkorolev/rocksdb,ylong/rocksdb,biddyweb/rocksdb,mbarbon/rocksdb,norton/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,makelivedotnet/rocksdb,tsheasha/rocksdb,mbarbon/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,luckywhu/rocksdb,makelivedotnet/rocksdb,amyvmiwei/rocksdb,lgscofield/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,kaschaeffer/rocksdb,IMCG/RcoksDB,flabby/rocksdb,hobinyoon/rocksdb,norton/rocksdb,caijieming-baidu/rocksdb,rDSN-Projects/rocksdb.replicated,IMCG/RcoksDB,OverlordQ/rocksdb,facebook/rocksdb,lgscofield/rocksdb,bbiao/rocksdb,wlqGit/rocksdb,wskplho/rocksdb,zhangpng/rocksdb,IMCG/RcoksDB,anagav/rocksdb,sorphi/rocksdb,temicai/rocksdb,wat-ze-hex/rocksdb,NickCis/rocksdb,lgscofield/rocksdb,wlqGit/rocksdb,JoeWoo/rocksdb,wlqGit/rocksdb,Applied-Duality/rocksdb,norton/rocksdb,hobinyoon/rocksdb,geraldoandradee/rocksdb,wskplho/rocksdb,bbiao/rocksdb,fengshao0907/rocksdb,RyanTech/rocksdb,geraldoandradee/rocksdb,Andymic/rocksdb,norton/rocksdb,amyvmiwei/rocksdb,siddhartharay007/rocksdb,anagav/rocksdb,facebook/rocksdb,Applied-Duality/rocksdb,wlqGit/rocksdb,norton/rocksdb,alihalabyah/rocksdb