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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48258

(cherry picked from commit 4940584bbf0b809130f6ac1a7a0c6b00d373af1e)