blk-mq: Fix the blk_mq_tagset_busy_iter() documentation

blk-mq: Fix the blk_mq_tagset_busy_iter() documentation

Commit 2dd6532e9591 ("blk-mq: Drop 'reserved' arg of busy_tag_iter_fn")
removed the 'reserved' argument from tag iteration callback functions.
Bring the blk_mq_tagset_busy_iter() documentation in sync with that
change.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>