net: ip: make ip_mc_validate_source() return drop reason

Make ip_mc_validate_source() return drop reason, and adjust the call of
it in ip_route_input_mc().

Another caller of it is ip_rcv_finish_core->udp_v4_early_demux, and the
errno is not checked in detail, so we don't do more adjustment for it.

The drop reason "SKB_DROP_REASON_IP_LOCALNET" is added in this commit.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>