ipv6_sock_mc_close() is called for ipv6 sockets at close time, and most
of them don't use multicast.

Add a test to avoid contention on a shared spinlock.

Same heuristic applies for ipv6_sock_ac_close(), to avoid contention
on a shared rwlock.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>