RDMA/rxe: Compute PSN windows correctly

RDMA/rxe: Compute PSN windows correctly

The code which limited the number of unacknowledged PSNs was incorrect.
The PSNs are limited to 24 bits and wrap back to zero from 0x00ffffff.
The test was computing a 32 bit value which wraps at 32 bits so that
qp->req.psn can appear smaller than the limit when it is actually larger.

Replace '>' test with psn_compare which is used for other PSN comparisons
and correctly handles the 24 bit size.

Fixes: 8700e3e7c485 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20201013170741.3590-1-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>