block: turn bdev_lock into a mutex

There is no reason for this lock to spin, and being able to sleep under
it will come in handy soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Link: https://lore.kernel.org/r/20230601094459.1350643-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>