selftests: mlxsw: Test FDB offload indication

Test that externally learned FDB entries added from user space are
marked as offloaded.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>