shared_mutex: reinstate FIFO ordering
shared_mutex: reinstate FIFO ordering

Noticed by Nadav.

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