md/bitmap: improve handling of 'allclean'.
The 'allclean' flag is used to cache the fact that there is nothing to do, so we can avoid waking up and scanning the bitmap regularly. The two sorts of pages that might need the attention of the bitmap daemon are BITMAP_PAGE_PENDING and BITMAP_PAGE_NEEDWRITE pages. So make sure allclean reflects exactly when there are none of those. So: set it before scanning all pages with either bit set. clear it whenever these bits are set clear it when we desire not to clear one of these bits. don't clear it any other time. Signed-off-by: NeilBrown <email@example.com>
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