IB/rxe: Add support for zero-byte operations

The last_psn algorithm fails in the zero-byte case: it calculates
first_psn = N, last_psn = N-1. This makes the operation unretryable since
the res structure will fail the (first_psn <= psn <= last_psn) test in
find_resource().

While here, use BTH_PSN_MASK to mask the calculated last_psn.

Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>