xen: fix bad grant operation
xen: fix bad grant operation

We used gnttab_grant_foreign_access() instead of
gnttab_grant_foreign_access_ref().  While the two functions have similar
enough signatures, they do very different things.

With the change, we are able to transmit packets through Xen.

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