fstream: fix write-behind on filesystems that don't support fallocate()
fstream: fix write-behind on filesystems that don't support fallocate()

Some filesystems don't support out variant of fallocate(), and our
implementation returns a ready future.  This can cause
complete_preallocation() to be called prematurely, modifying the
variables used in the loop, confusing it.

Fix be restarting the loop in this case with a goto.  Ugly, will improve
later.

leejir/seastar,chunshengster/seastar,jonathanleang/seastar,linearregression/seastar,avikivity/seastar,hongliangzhao/seastar,syuu1228/seastar,acbellini/seastar,wildinto/seastar,chunshengster/seastar,printedheart/seastar,dreamsxin/seastar,stamhe/seastar,anzihenry/seastar,shyamalschandra/seastar,norcimo5/seastar,scylladb/seastar,dwdm/seastar,anzihenry/seastar,hongliangzhao/seastar,scylladb/scylla-seastar,bowlofstew/seastar,flashbuckets/seastar,printedheart/seastar,scylladb/seastar,stamhe/seastar,jonathanleang/seastar,joerg84/seastar,norcimo5/seastar,dreamsxin/seastar,kjniemi/seastar,scylladb/seastar,scylladb/scylla-seastar,flashbuckets/seastar,sjperkins/seastar,cloudius-systems/seastar,tempbottle/seastar,wildinto/seastar,chunshengster/seastar,cloudius-systems/seastar,stamhe/seastar,shyamalschandra/seastar,dwdm/seastar,acbellini/seastar,sjperkins/seastar,wildinto/seastar,scylladb/scylla-seastar,koolhazz/seastar,cloudius-systems/seastar,dwdm/seastar,koolhazz/seastar,syuu1228/seastar,bowlofstew/seastar,joerg84/seastar,dreamsxin/seastar,acbellini/seastar,sjperkins/seastar,printedheart/seastar,xtao/seastar,avikivity/seastar,leejir/seastar,linearregression/seastar,flashbuckets/seastar,avikivity/seastar,tempbottle/seastar,leejir/seastar,xtao/seastar,tempbottle/seastar,xtao/seastar,anzihenry/seastar,kjniemi/seastar,shyamalschandra/seastar,hongliangzhao/seastar,joerg84/seastar,kjniemi/seastar,bowlofstew/seastar,norcimo5/seastar,koolhazz/seastar,linearregression/seastar,syuu1228/seastar,jonathanleang/seastar