reactor: workaround missing FALLOC_FL_ZERO_RANGE in kernel headers
reactor: workaround missing FALLOC_FL_ZERO_RANGE in kernel headers

Prehistoric kernels don't expose FALLOC_FL_ZERO_RANGE, humor them.

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