IB/mlx4: Verify net device validity on port change event

Processing an event is done in a different context from the one when
the event was dispatched. This requires a check that the slave
net device is still valid when the event is being processed. The check is done
under the iboe lock which ensure correctness.

Fixes: a5750090 ('IB/mlx4: Add port aggregation support')
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>