net: fix dpdk bulk sending
net: fix dpdk bulk sending

_tx_burst_idx may be zero, but _tx_burst still holds packets from the top
of circular_buffer, so avoid refilling in this case.

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