md/md-llbitmap: clear flush state after daemon flush

llbitmap_flush() sets LLPageFlush on each bitmap page before it queues the
daemon worker. The flag tells md_llbitmap_daemon_fn() to ignore the normal
barrier_idle expiry check and clean the page immediately.

The daemon only tested LLPageFlush. Once a page had been flushed explicitly,
the flag stayed set, so later dirty bits on that page also bypassed
barrier_idle and were cleaned the next time the daemon ran. That can make a
new write look clean much earlier than the configured idle window.

Consume LLPageFlush in md_llbitmap_daemon_fn() with test_and_clear_bit() and
use the returned value for the current expiry check. The explicit flush still
forces the current daemon pass, while later writes on the same page wait for
barrier_idle again.

This can be reproduced through normal sysfs operations:

  1. Create a small RAID1 with --bitmap=lockless and --assume-clean.
  2. Set llbitmap/daemon_sleep=1 and llbitmap/barrier_idle=10.
  3. Toggle md/array_state from active to readonly and back to active to call
     llbitmap_flush() without destroying the in-memory bitmap.
  4. Write one sector and read llbitmap/bits immediately, after 2 seconds,
     and after 12 seconds.

On the bad kernel the dirty bit is already clean after 2 seconds. With this
change it remains dirty until the barrier_idle window expires.

Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-2-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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