Fix integer overflow in folly/Benchmark.cpp
Fix integer overflow in folly/Benchmark.cpp

Summary:
`bm_max_secs` has type int32
So when this value is more than 2, expression
```
FLAGS_bm_max_secs * 1000000000;
```
overflows

Test Plan:
I did put a
```
LOG(INFO) << timeBudgetInNs;
```
and observed correct values (before fix they were overflowed)

Reviewed By: antonl@fb.com

Subscribers: njormrod, folly-diffs@

FB internal diff: D1663247

Signature: t1:1663247:1415261814:c9154ffde183b2a4f5403e534e47e52e8276c61b

clearlylin/folly,hongliangzhao/folly,zhiweicai/folly,romange/folly,doctaweeks/folly,mqeizi/folly,sakishum/folly,clearlinux/folly,cole14/folly,juniway/folly,facebook/folly,bsampath/folly,stonegithubs/folly,yangjin-unique/folly,PPC64/folly,project-zerus/folly,yangjin-unique/folly,colemancda/folly,brunomorishita/folly,wildinto/folly,SeanRBurton/folly,guker/folly,hongliangzhao/folly,PoisonBOx/folly,hongliangzhao/folly,tempbottle/folly,leolujuyi/folly,Orvid/folly,charsyam/folly,wildinto/folly,fw1121/folly,cole14/folly,bikong2/folly,lifei/folly,cole14/folly,rklabs/folly,renyinew/folly,cppfool/folly,cole14/folly,tomhughes/folly,Hincoin/folly,sakishum/folly,PPC64/folly,cppfool/folly,colemancda/folly,reddit/folly,charsyam/folly,kernelim/folly,floxard/folly,doctaweeks/folly,hongliangzhao/folly,fw1121/folly,bikong2/folly,tempbottle/folly,leolujuyi/folly,PoisonBOx/folly,arg0/folly,guker/folly,gavioto/folly,mqeizi/folly,romange/folly,floxard/folly,colemancda/folly,chjp2046/folly,ykarlsbrun/folly,CJstar/folly,clearlylin/folly,constantine001/folly,renyinew/folly,gavioto/folly,raphaelamorim/folly,mqeizi/folly,Orvid/folly,tempbottle/folly,renyinew/folly,lifei/folly,SammyK/folly,loverszhaokai/folly,rklabs/folly,tomhughes/folly,brunomorishita/folly,PoisonBOx/folly,rklabs/folly,clearlinux/folly,chjp2046/folly,nickhen/folly,shaobz/folly,arg0/folly,tempbottle/folly,shaobz/folly,upsoft/folly,theiver9827/folly,arg0/folly,chjp2046/folly,reddit/folly,floxard/folly,Eagle-X/folly,stonegithubs/folly,KSreeHarsha/folly,arg0/folly,doctaweeks/folly,floxard/folly,wildinto/folly,project-zerus/folly,constantine001/folly,alexst07/folly,tomhughes/folly,zhiweicai/folly,CJstar/folly,SammyK/folly,facebook/folly,gaoyingie/folly,kernelim/folly,bikong2/folly,Hincoin/folly,stonegithubs/folly,theiver9827/folly,Orvid/folly,SeanRBurton/folly,xzmagic/folly,sonnyhu/folly,sakishum/folly,reddit/folly,arg0/folly,PPC64/folly,xzmagic/folly,bowlofstew/folly,wildinto/folly,KSreeHarsha/folly,nickhen/folly,clearlinux/folly,juniway/folly,bsampath/folly,shaobz/folly,gaoyingie/folly,lifei/folly,xzmagic/folly,xzmagic/folly,bikong2/folly,reddit/folly,colemancda/folly,nickhen/folly,guker/folly,bowlofstew/folly,SeanRBurton/folly,bowlofstew/folly,Orvid/folly,clearlinux/folly,renyinew/folly,bobegir/folly,raphaelamorim/folly,tempbottle/folly,bowlofstew/folly,Eagle-X/folly,clearlylin/folly,SammyK/folly,loversInJapan/folly,doctaweeks/folly,brunomorishita/folly,leolujuyi/folly,romange/folly,mqeizi/folly,alexst07/folly,renyinew/folly,gavioto/folly,bobegir/folly,reddit/folly,bikong2/folly,fw1121/folly,yangjin-unique/folly,bsampath/folly,sonnyhu/folly,nickhen/folly,sakishum/folly,loverszhaokai/folly,lifei/folly,Hincoin/folly,theiver9827/folly,KSreeHarsha/folly,bsampath/folly,stonegithubs/folly,PoisonBOx/folly,sonnyhu/folly,leolujuyi/folly,PPC64/folly,gavioto/folly,Eagle-X/folly,facebook/folly,gavioto/folly,loverszhaokai/folly,raphaelamorim/folly,yangjin-unique/folly,rklabs/folly,sonnyhu/folly,bobegir/folly,juniway/folly,ykarlsbrun/folly,Hincoin/folly,upsoft/folly,zhiweicai/folly,upsoft/folly,guker/folly,mqeizi/folly,SeanRBurton/folly,fw1121/folly,project-zerus/folly,Hincoin/folly,theiver9827/folly,gaoyingie/folly,romange/folly,colemancda/folly,constantine001/folly,alexst07/folly,raphaelamorim/folly,romange/folly,guker/folly,xzmagic/folly,cppfool/folly,ykarlsbrun/folly,SammyK/folly,loversInJapan/folly,zhiweicai/folly,charsyam/folly,doctaweeks/folly,clearlylin/folly,bsampath/folly,gaoyingie/folly,loversInJapan/folly,tomhughes/folly,cole14/folly,hongliangzhao/folly,sonnyhu/folly,cppfool/folly,gaoyingie/folly,brunomorishita/folly,leolujuyi/folly,loverszhaokai/folly,zhiweicai/folly,yangjin-unique/folly,upsoft/folly,facebook/folly,alexst07/folly,CJstar/folly,bowlofstew/folly,charsyam/folly,bobegir/folly,SeanRBurton/folly,ykarlsbrun/folly,bobegir/folly,shaobz/folly,loversInJapan/folly,brunomorishita/folly,fw1121/folly,ykarlsbrun/folly,wildinto/folly,rklabs/folly,tomhughes/folly,CJstar/folly,sakishum/folly,project-zerus/folly,upsoft/folly,PoisonBOx/folly,loversInJapan/folly,facebook/folly,Eagle-X/folly,alexst07/folly,Orvid/folly,constantine001/folly,KSreeHarsha/folly,stonegithubs/folly,juniway/folly,lifei/folly,constantine001/folly,KSreeHarsha/folly,kernelim/folly,PPC64/folly,floxard/folly,clearlylin/folly,Eagle-X/folly,chjp2046/folly,theiver9827/folly,juniway/folly,raphaelamorim/folly,loverszhaokai/folly,project-zerus/folly,kernelim/folly,cppfool/folly,nickhen/folly,kernelim/folly,SammyK/folly,CJstar/folly,chjp2046/folly,shaobz/folly,clearlinux/folly