nbd: Improve the documentation of the locking assumptions

Document locking assumptions with lockdep_assert_held() instead of source
code comments. The advantage of lockdep_assert_held() is that it is
verified at runtime if lockdep is enabled in the kernel config.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510202313.25209-4-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -588,7 +588,6 @@ static inline int was_interrupted(int result)
 	return result == -ERESTARTSYS || result == -EINTR;
 }
 
-/* always call with the tx_lock held */
 static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 {
 	struct request *req = blk_mq_rq_from_pdu(cmd);
@@ -605,6 +604,9 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 	u32 nbd_cmd_flags = 0;
 	int sent = nsock->sent, skip = 0;
 
+	lockdep_assert_held(&cmd->lock);
+	lockdep_assert_held(&nsock->tx_lock);
+
 	iov_iter_kvec(&from, ITER_SOURCE, &iov, 1, sizeof(request));
 
 	type = req_to_nbd_cmd_type(req);
@@ -1015,6 +1017,8 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
 	struct nbd_sock *nsock;
 	int ret;
 
+	lockdep_assert_held(&cmd->lock);
+
 	config = nbd_get_config_unlocked(nbd);
 	if (!config) {
 		dev_err_ratelimited(disk_to_dev(nbd->disk),