Improve two KASSERTs in the TCP RACK stack.

There are two locations where an always true comparison was made in
a KASSERT. Replace this by an appropriate check and use a consistent
panic message. Also use this code when checking a similar condition.

PR: 229664
Reviewed by: rrs@
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D18021