bridge: Add core IGMP snooping support

This patch adds the core functionality of IGMP snooping support
without actually hooking it up.  So this patch should be a no-op
as far as the bridge's external behaviour is concerned.

All the new code and data is controlled by the Kconfig option
BRIDGE_IGMP_SNOOPING.  A run-time toggle is also available.

The multicast switching is done using an hash table that is
lockless on the read-side through RCU.  On the write-side the
new multicast_lock is used for all operations.  The hash table
supports dynamic growth/rehashing.

The hash table will be rehashed if any chain length exceeds a
preset limit.  If rehashing does not reduce the maximum chain
length then snooping will be disabled.

These features may be added in future (in no particular order):

* IGMPv3 source support
* Non-querier router detection
* IPv6

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