bridge: multicast: fix handling of temp and perm entries

When the bridge (or port) is brought down/up flush only temp entries and
leave the perm ones. Flush perm entries only when deleting the bridge
device or the associated port.

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>