IB/core: Make sure that PSN does not overflow

The rq/sq->psn is 24 bits as defined in the IB spec, therefore we mask
out the 8 most significant bits to avoid overflow in modify_qp.

Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>