nbd: Improve the documentation of the locking assumptions

[ Upstream commit 2a6751e0

 ]

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.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: e56d4b63

 ("nbd: Fix signal handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>