sched/core: Account I/O waits independently of delayed dequeue

Starting blkio delay accounting and incrementing rq->nr_iowait in
__block_task() ties both to removal from the runqueue. With EEVDF delayed
dequeue, this misses the initial part of an I/O sleep, or the entire
sleep if the task wakes before it is dequeued. Such tasks are also
missing from procs_blocked while their dequeue is delayed. Final dequeue
can run in another task's context, causing delayacct_blkio_start() to
timestamp the wrong task.

Start both forms of accounting in try_to_block_task(), when the current
task commits to blocking, before delayed dequeue or proxy execution can
retain it on the runqueue. End both when clearing is_blocked, including
queued wakeups and the proxy execution paths that let the current task
continue. Checking is_blocked excludes wakeups that precede blocking.

Save the CPU charged at the start of the wait. Delayed tasks can migrate
while still queued, and proxy execution can also move blocked tasks.
Decrement the saved CPU's counter on unblock, regardless of the task's
current placement, and remove the old dequeue-based accounting.

CPU iowait continues to classify idle time using the counter on the
charged CPU; it does not accumulate the full per-task blocked duration.

Closes: https://lore.kernel.org/r/20261014081731.1068794-1-zhanxusheng@xiaomi.com
Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Link: https://patch.msgid.link/fd8578a1-e60b-4993-ab51-2b5d63abcf77@arm.com

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