bridge: mdb: notify on router port add and del

Send notifications on router port add and del/expire, re-use the already
existing MDBA_ROUTER and send NEWMDB/DELMDB netlink notifications
respectively.

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