RDMA/rxe: Remove qp->req.state

The rxe driver has four different QP state variables,
    qp->attr.qp_state,
    qp->req.state,
    qp->comp.state, and
    qp->resp.state.
All of these basically carry the same information.

This patch replaces uses of qp->req.state by qp->attr.qp_state and enum
rxe_qp_state.  This is the third of three patches which will remove all
but the qp->attr.qp_state variable. This will bring the driver closer to
the IBA description.

Link: https://lore.kernel.org/r/20230405042611.6467-3-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>