IB/mlx4: Rework special QP creation error path

The special QP creation error path relies on offset_of(struct mlx4_ib_sqp,
qp) == 0. Remove this assumption because that makes the QP creation
code easier to understand.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>