IB/core: Perform modify QP on real one

Currently qp->port stores the port number whenever IB_QP_PORT
QP attribute mask is set (during QP state transition to INIT state).
This port number should be stored for the real QP when XRC target QP
is used.

Follow the ib_modify_qp() implementation and hide the access to ->real_qp.

Fixes: a512c2fb

 ("IB/core: Introduce modify QP operation with udata")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>