bridge: don't age externally added FDB entries

Signed-off-by: Siva Mannem <siva.mannem.lnx@gmail.com>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Premkumar Jonnala <pjonnala@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>