Always use inp fib in the inp_lookup_mcast_ifp().

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