IB/mlx4: Fix VF mac handling in RoCE

We had several problems here.  First, a race condition on QP1 mac
handling between mlx4_ib_update_qps and mlx4_ib_modify_qp, which is
fixed by taking the qp mutex in mlx4_ib_update_qps.

Also, qp->pri.smac_port was not updated in mlx4_ib_update_qps.

Last, in __mlx4_ib_modify_qp we did not properly handle the case where
the mac is zero, but port is non-zero.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>