nvme: allocate the keep alive request using BLK_MQ_REQ_NOWAIT

To avoid an error recovery deadlock where the keep alive work is waiting
for a request and thus can't be flushed to make progress for tearing down
the controller.  Also print the error code returned from
blk_mq_alloc_request to help debugging any future issues in this code.

Based on an earlier patch from Hannes Reinecke <hare@suse.de>.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Daniel Wagner <dwagner@suse.de>

drivers/nvme/host/core.c

@@ -1242,10 +1242,10 @@ static void nvme_keep_alive_work(struct work_struct *work)
 	}
 
 	rq = nvme_alloc_request(ctrl->admin_q, &ctrl->ka_cmd,
-				BLK_MQ_REQ_RESERVED);
+				BLK_MQ_REQ_RESERVED | BLK_MQ_REQ_NOWAIT);
 	if (IS_ERR(rq)) {
 		/* allocation failure, reset the controller */
-		dev_err(ctrl->device, "keep-alive failed\n");
+		dev_err(ctrl->device, "keep-alive failed: %ld\n", PTR_ERR(rq));
 		nvme_reset_ctrl(ctrl);
 		return;
 	}