net: ipv4: make udp_v4_early_demux explicitly return drop reason

net: ipv4: make udp_v4_early_demux explicitly return drop reason

udp_v4_early_demux already returns drop reasons as it either returns 0
or ip_mc_validate_source, which itself returns drop reasons. Its return
value is also already used as a drop reason itself.

Makes this explicit by making it return drop reasons.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250915091958.15382-2-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>