Skyzaller crash found by Michael Tuexen

[rrs]

Summary

Michael was able to get skyzaller to cause rack to crash with a lot of strange 0 byte send map entries. This
is because the snd_nxt was not being properly handled in the non FO case. This fixes that and
provides a small tweak to BBR as well (though I don't think this is the root cause of the bbr
Michael ran across too).

Test Plan

Run skyzaller for +100 hours to make sure it does not "rediscover" the bug/crash.

[val_packett.cool]

(syzkaller, not skyzaller. it does syztem kalls. nothing to do with the sky :D)

[tuexen]

Question: You are introducting the flag RACK_HAS_SIN and you are setting it. But you are never reading it. Is that intended?

[rrs]

Ok turns out skyzaller has found yet another crash having to do with TFO for all three stacks. Lets update
the patch to fix that too.