udp: Simplify __udp*_lib_mcast_deliver.

Switch to using sk_nulls_for_each which shortens the code and makes it
easier to update.

Signed-off-by: David Held <drheld@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>