added_by_external_learn fdb entries are added and expired by
external entities like switchdev driver or external controllers.
ageing is already disabled for such entries. Hence, don't
indicate expiry for such fdb entries.

CC: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
CC: Jiri Pirko <jiri@resnulli.us>
CC: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: NIdo Schimmel <idosch@mellanox.com>
Tested-by: NIdo Schimmel <idosch@mellanox.com>
Reviewed-by: NNikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>