bridge: mcast: add support for temporary port router

Add support for a temporary router port which doesn't depend only on the
incoming query. It can be refreshed if set to the same value, which is
a no-op for the rest.

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