IB/rxe: Cache dst in QP instead of getting it for each send

In RC QP there is no need to resolve the outgoing interface
for each packet, as this does not change during QP life cycle.

Instead cache the interface on the socket and use that one.
This improves performance by 12% by sparing redundant
calls to rxe_find_route.

ib_send_bw -d rxe0  -x 1 -n 9000 -e  -s $((1024 * 1024 )) -l 100

----------------------------------------------------------------------------------------
|        | bytes   | iterations | BW peak[MB/sec] | BW average[MB/sec] | MsgRate[Mpps] |
----------------------------------------------------------------------------------------
| before | 1048576 | 9000       | inf             | 551.21             | 0.000551      |
| after  | 1048576 | 9000       | inf             | 615.54             | 0.000616      |
----------------------------------------------------------------------------------------

Fixes: 8700e3e7 ("Soft RoCE driver")
Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>