net: bridge: convert multicast to generic rhashtable

The bridge multicast code currently uses a custom resizable hashtable
which predates the generic rhashtable interface. It has many
shortcomings compared and duplicates functionality that is presently
available via the generic rhashtable, so this patch removes the custom
rhashtable implementation in favor of the kernel's generic rhashtable.
The hash maximum is kept and the rhashtable's size is used to do a loose
check if it's reached in which case we revert to the old behaviour and
disable further bridge multicast processing. Also now we can support any
hash maximum, doesn't need to be a power of 2.

v3: add non-rcu br_mdb_get variant and use it where multicast_lock is
    held to avoid RCU splat, drop hash_max function and just set it
    directly

v2: handle when IGMP snooping is undefined, add br_mdb_init/uninit
    placeholders

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>