TCP RACK, BBR: cleanup of ctf_process_inbound_raw()

Instead of dealing with ifp == NULL, which should never happen,
assume that this is not true. Use KASSERT to make this clear.
No functional change intended.

Reviewed by: glebius, rrs
CID: 1523767
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48258