file: optimize file::size() for posix
file: optimize file::size() for posix

lseek() won't block, so use it for fetching the size of a file directly
from the reactor thread, instead of an iothread.

rluta/scylla,aruanruan/scylla,acbellini/seastar,stamhe/scylla,gwicke/scylla,dwdm/seastar,norcimo5/seastar,syuu1228/seastar,justintung/scylla,tempbottle/seastar,cloudius-systems/seastar,acbellini/seastar,bowlofstew/scylla,eklitzke/scylla,dwdm/seastar,bzero/seastar,linearregression/scylla,joerg84/seastar,raphaelsc/scylla,stamhe/seastar,flashbuckets/seastar,aruanruan/scylla,linearregression/seastar,leejir/seastar,victorbriz/scylla,respu/scylla,hongliangzhao/seastar,respu/scylla,senseb/scylla,guiquanz/scylla,rentongzhang/scylla,rentongzhang/scylla,kjniemi/scylla,linearregression/scylla,joerg84/seastar,glommer/scylla,cloudius-systems/seastar,koolhazz/seastar,chunshengster/seastar,hongliangzhao/seastar,aruanruan/scylla,tempbottle/scylla,mixja/seastar,tempbottle/seastar,gwicke/scylla,dreamsxin/seastar,flashbuckets/seastar,acbellini/scylla,wildinto/seastar,dreamsxin/seastar,linearregression/seastar,avikivity/scylla,wildinto/scylla,norcimo5/seastar,mixja/seastar,wildinto/scylla,guiquanz/scylla,respu/scylla,syuu1228/seastar,stamhe/seastar,justintung/scylla,koolhazz/seastar,acbellini/scylla,wildinto/seastar,shyamalschandra/seastar,scylladb/scylla-seastar,asias/scylla,gwicke/scylla,sjperkins/seastar,leejir/seastar,rluta/scylla,sjperkins/seastar,kjniemi/scylla,eklitzke/scylla,avikivity/scylla,leejir/seastar,rluta/scylla,jonathanleang/seastar,shyamalschandra/seastar,stamhe/scylla,cloudius-systems/seastar,phonkee/scylla,shaunstanislaus/scylla,tempbottle/scylla,capturePointer/scylla,kjniemi/seastar,duarten/scylla,printedheart/seastar,syuu1228/seastar,norcimo5/seastar,bzero/seastar,joerg84/seastar,chunshengster/seastar,sjperkins/seastar,asias/scylla,rentongzhang/scylla,bowlofstew/seastar,avikivity/seastar,bowlofstew/seastar,duarten/scylla,phonkee/scylla,asias/scylla,jonathanleang/seastar,scylladb/scylla,guiquanz/scylla,glommer/scylla,wildinto/scylla,dreamsxin/seastar,raphaelsc/scylla,xtao/seastar,flashbuckets/seastar,dwdm/scylla,eklitzke/scylla,avikivity/seastar,kangkot/scylla,koolhazz/seastar,dwdm/scylla,justintung/scylla,scylladb/scylla,anzihenry/seastar,scylladb/scylla-seastar,bowlofstew/scylla,wildinto/seastar,stamhe/scylla,senseb/scylla,shaunstanislaus/scylla,dwdm/seastar,scylladb/seastar,scylladb/scylla,acbellini/scylla,printedheart/seastar,acbellini/seastar,kjniemi/scylla,glommer/scylla,shyamalschandra/seastar,xtao/seastar,victorbriz/scylla,tempbottle/scylla,mixja/seastar,anzihenry/seastar,anzihenry/seastar,kjniemi/seastar,printedheart/seastar,capturePointer/scylla,duarten/scylla,raphaelsc/scylla,phonkee/scylla,avikivity/seastar,scylladb/seastar,scylladb/seastar,kangkot/scylla,dwdm/scylla,scylladb/scylla,kangkot/scylla,linearregression/scylla,capturePointer/scylla,chunshengster/seastar,bowlofstew/scylla,hongliangzhao/seastar,stamhe/seastar,avikivity/scylla,linearregression/seastar,jonathanleang/seastar,bowlofstew/seastar,shaunstanislaus/scylla,bzero/seastar,senseb/scylla,kjniemi/seastar,tempbottle/seastar,scylladb/scylla-seastar,xtao/seastar,victorbriz/scylla