blk-mq: Defer freeing flush queue to SRCU callback

The freeing of the flush queue/request in blk_mq_exit_hctx() can race with
tag iterators that may still be accessing it. To prevent a potential
use-after-free, the deallocation should be deferred until after a grace
period. With this way, we can replace the big tags->lock in tags iterator
code path with srcu for solving the issue.

This patch introduces an SRCU-based deferred freeing mechanism for the
flush queue.

The changes include:
- Adding a `rcu_head` to `struct blk_flush_queue`.
- Creating a new callback function, `blk_free_flush_queue_callback`,
  to handle the actual freeing.
- Replacing the direct call to `blk_free_flush_queue()` in
  `blk_mq_exit_hctx()` with `call_srcu()`, using the `tags_srcu`
  instance to ensure synchronization with tag iterators.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>