IB/rxe: Don't update the response PSN unless it's going forwards

A client might post a read followed by a send. The partner receives
and acknowledges both transactions, posting an RCQ entry for the
send, but something goes wrong with the read ACK. When the client
retries the read, the partner's responder processes the duplicate
read but incorrectly resets the PSN to the value preceding the
original send. When the duplicate send arrives, the responder cannot
tell that it is a duplicate, so the responder generates a duplicate
RCQ entry, confusing the client.

Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Reviewed-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>