IB/rxe: Clear queue buffer when modifying QP to reset

RXE resets the send-q only once in rxe_qp_init_req() when
QP is created, but when the QP is reused after QP reset, the send-q
holds previous garbage data.

This garbage data wrongly fails CQEs that otherwise
should have completed successfully.

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