vhost-scsi: Check for a cleared backend before queueing an event

vhost-scsi: Check for a cleared backend before queueing an event

We currenly hold the vhost_scsi_mutex while clearing the endpoint and
while performing vhost_scsi_do_plug, so tpg->vhost_scsi can't be freed
from uder us, and to make sure anything queued is handled by the
full call in vhost_scsi_clear_endpoint.

This patch removes the need for the vhost_scsi_mutex for the latter
case. In the next patches, we won't hold the vhost_scsi_mutex while
flushing so this patch adds a check for the clearing of the virtqueue
from vhost_scsi_clear_endpoint. We then know that once
vhost_scsi_clear_endpoint has cleared the backend that no new events
will be queued, and the flush after the vhost_vq_set_backend(vq, NULL)
call will see everything that's been queued to that point. So the flush
will then handle all events without the need for the vhost_scsi_mutex.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230321020624.13323-6-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>