blk-zoned: Document locking assumptions

Document which functions expect that their callers must hold a lock.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241217210310.645966-3-bvanassche@acm.org


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