net: remove WARN_ON_ONCE() from sk_mc_loop()

net: remove WARN_ON_ONCE() from sk_mc_loop()

sk_mc_loop() can be called for sockets that are neither AF_INET
nor AF_INET6 (e.g. AF_PACKET sockets when sending packets via raw/packet
socket over virtual devices such as VRF or ipvlan).

In such cases, sk_family is not AF_INET/AF_INET6 and sk_mc_loop() falls
through the switch statement and triggers WARN_ON_ONCE(1).

Non-INET sockets do not support IP_MULTICAST_LOOP or IPV6_MULTICAST_LOOP
options, so loopback should default to true without generating a warning.

Fixes: f60e5990d9c1 ("ipv6: protect skb->sk accesses from recursive dereference inside the stack")
Reported-by: syzbot+22c3218a6fa219e47321@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a72024c.13623e66.bdc14.0019.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260804152048.2134341-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>