tcp: properly calculate the payload length for TSO decision
tcp: properly calculate the payload length for TSO decision

Fixes issue #41

Packet doesn't have neither L2 nor L3 headers in the tcp::output_one(),
therefore comparing p.len() to MSS + L2_HDR + L3_HDR + L4_HDR when
payload size is greater than (MSS - L2_HDR - L3_HDR) gives the wrong
negative decision in regard to whether to send this frame as an TSO frame.

Fix the above by comparing the MSS to actual payload size.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>
Reviewed-by: Asias He <7487d0432e24c09ba3f2ff7de44b9eb030d5ae18@cloudius-systems.com>

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