net: sprinkle __predict_false in ip_input on error conditions

While here rearrange the RVSP check to inspect proto first and avoid
evaluating V_rsvp in the common case to begin with (most notably avoid
the expensive read).

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32929