TCP: Fix a rack bug that skyzall found which results in a crash.

So when we call the fast_rsm retransmit path, we should always move
snd_nxt back up to snd_max. In fact during ack-processing if snd_nxt
falls behind it should be moved up there as well. Otherwise what
can happen is we have an incorrect mark on snd_nxt and incorrectly
calculate the offset when we go through the front path (which is
what skzyall was able to do) then when we go to clean up the
send the offset is all wrong and we crash.

Special thanks to Gleb for pointing out the problem and the email
that had the reproducer so I could find the issue.

Reported-by: syzbot+f5061a372f74f021ec02@syzkaller.appspotmail.com
Sponsored by: Netflix Inc