bridge: allow adding of fdb entries pointing to the bridge device

This patch enables adding of fdb entries pointing to the bridge device.
This can be used to propagate mac address of vlan interfaces
configured on top of the vlan filtering bridge.

Before:
$bridge fdb add 44:38:39:00:27:9f dev bridge
RTNETLINK answers: Invalid argument

After:
$bridge fdb add 44:38:39:00:27:9f dev bridge

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