net: bridge: optimize backup_port fdb convergence

We can optimize the fdb convergence when a backup_port is present by not
immediately flushing the entries of the stopped port since traffic for
those entries will flow towards the backup_port.

There are 2 cases specifically that benefit most:
- when the stopped port comes up before the entries expire by themselves
- when there's an external entry refresh and they're kept while the
  backup_port is operating (e.g. mlag)

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