So it turns out that syzkaller hit another crash. It has to do with switching
stacks with a SENT_FIN outstanding. Both rack and bbr will only send a
FIN if all data is ack'd so this must be enforced. Also if the previous stack
sent the FIN we need to make sure in rack that when we manufacture the
"unknown" sends that we include the proper HAS_FIN bits.

Note for BBR we take a simpler approach and just refuse to switch.

Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D26269