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.