Always use inp fib in the inp_lookup_mcast_ifp().

inp_lookup_mcast_ifp() is static and is only used in the inp_join_group().
The latter function is also static, and is only used in the inp_setmoptions(),
which relies on inp being non-NULL.

As a result, in the current code, inp_lookup_mcast_ifp() is always called
with non-NULL inp. Eliminate unused RT_DEFAULT_FIB condition and always
use inp fib instead.

Differential Revision: https://reviews.freebsd.org/D29594
Reviewed by: kp
MFC after: 2 weeks

(cherry picked from commit c3a456defaf28df8d6f47704d606248f6c22146c)