md: add check for sleepers in md_wakeup_thread()

Check for sleeping thread before attempting its wake_up in
md_wakeup_thread() to avoid unnecessary spinlock contention.

With a 6.1 kernel, fio random read/write tests on many (>= 100)
virtual volumes, of 100 GiB each, on 3 md-raid5s on 8 SSDs each
(building a raid50), show by 3 to 4 % improved IOPS performance.

Signed-off-by: Florian-Ewald Mueller <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Signed-off-by: Jack Wang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Song Liu <[email protected]>

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