RDMA/rxe: Fix a race condition related to the QP error state

RDMA/rxe: Fix a race condition related to the QP error state

The following sequence:
* Change queue pair state into IB_QPS_ERR.
* Post a work request on the queue pair.

Triggers the following race condition in the rdma_rxe driver:
* rxe_qp_error() triggers an asynchronous call of rxe_completer(), the function
  that examines the QP send queue.
* rxe_post_send() posts a work request on the QP send queue.

If rxe_completer() runs prior to rxe_post_send(), it will drain the send
queue and the driver will assume no further action is necessary.
However, once we post the send to the send queue, because the queue is
in error, no send completion will ever happen and the send will get
stuck.  In order to process the send, we need to make sure that
rxe_completer() gets run after a send is posted to a queue pair in an
error state.  This patch ensures that happens.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Moni Shoua <monis@mellanox.com>
Cc: <stable@vger.kernel.org> # v4.8
Signed-off-by: Doug Ledford <dledford@redhat.com>