tcp: stack unloading crash in rack and bbr

Its possible to induce a crash in either rack or bbr. This would be done
if the rack stack were say the default and bbr was being used by a connection.
If the bbr stack is then unloaded and it was active, we will trigger a MPASS assert
in tcp_hpts since the new stack (default rack) would start a timer, and the old stack
(bbr) would have the inp already in hpts.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39576