net/mlx4: Add reference counting to MAC registeration

Add reference counting to the driver MAC registeration code. This would
be needed for cases where a mac is registered from more than once, e.g
when both the host and the VM driver register the same mac, the host
for mac spoof protection purposes and the VM for its regular needs.

Signed-off-by: Rony Efraim <ronye@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>