net: bridge: multicast: factor out port multicast context

Factor out the port's multicast context into a separate structure which
will later be shared for per-port,vlan context. No functional changes
intended. We need the structure even if bridge multicast is not defined
to pass down as pointer to forwarding functions.

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