NVMe: Add the nvme thread to the wait queue before waking it up

If the I/O was not completed by a single NVMe command, we add the
bio to the congestion list and wake up the kthread to resubmit it.
But the kthread calls remove_wait_queue() unconditionally, which
will oops if it's not on the wait queue.  So add the kthread to
the wait queue before waking it up.
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme.c

@@ -325,6 +325,8 @@ static void bio_completion(struct nvme_queue *nvmeq, void *ctx,
 	if (status) {
 		bio_endio(bio, -EIO);
 	} else if (bio->bi_vcnt > bio->bi_idx) {
+		if (bio_list_empty(&nvmeq->sq_cong))
+			add_wait_queue(&nvmeq->sq_full, &nvmeq->sq_cong_wait);
 		bio_list_add(&nvmeq->sq_cong, bio);
 		wake_up_process(nvme_thread);
 	} else {