bridge: Add multicast forwarding functions

This patch adds code to perform selective multicast forwarding.

We forward multicast traffic to a set of ports plus all multicast
router ports.  In order to avoid duplications among these two
sets of ports, we order all ports by the numeric value of their
pointers.  The two lists are then walked in lock-step to eliminate
duplicates.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>