net: fix IP checksum overflow during reduction
net: fix IP checksum overflow during reduction

When reducing the checksum from a 32-bit or 64-bit intermediate,
we can get an overflow after the first overflow handling step:

0000_8000_8000_ffff
-> 10_ffff
->  1_000f
->    0010

Since we lacked the second step, we got an off-by-one in the checksum.

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