xen: fix memory barrier when writing rx buffer ring
xen: fix memory barrier when writing rx buffer ring

The barrier must separate writing the ring data from the ring index,
otherwise the other side may see unwritten ring data.

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