blk-mq: Document tags_srcu member in blk_mq_tag_set structure

JIRA: https://issues.redhat.com/browse/RHEL-123480

commit 199c9a8
Author: Ming Lei <ming.lei@redhat.com>
Date:   Tue Sep 9 20:33:10 2025 +0800

    blk-mq: Document tags_srcu member in blk_mq_tag_set structure

    Add missing documentation for the tags_srcu member that was introduced
    to defer freeing of tags page_list to prevent use-after-free when
    iterating tags.

    Fixes htmldocs warning:
    WARNING: include/linux/blk-mq.h:536 struct member 'tags_srcu' not described in 'blk_mq_tag_set'

    Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

include/linux/blk-mq.h

@@ -518,6 +518,8 @@ enum hctx_type {
  *		   request_queue.tag_set_list.
  * @srcu:	   Use as lock when type of the request queue is blocking
  *		   (BLK_MQ_F_BLOCKING).
+ * @tags_srcu:	   SRCU used to defer freeing of tags page_list to prevent
+ *		   use-after-free when iterating tags.
  * @update_nr_hwq_lock:
  * 		   Synchronize updating nr_hw_queues with add/del disk &
  * 		   switching elevator.