`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.