btrfs: unify folio dirty flag clearing

Currently during folio writeback, we call folio_clear_dirty_for_io()
before extent_writepage(), which causes folio dirty flag to be cleared,
but without touching the subpage bitmaps.

This works fine for the bio submission path, as we always call
btrfs_folio_clear_dirty() to clear the subpage bitmap.

But this is far from consistent, thus this patch is going to unify the
behavior to always use btrfs_folio_clear_dirty() helper to clear both
folio flag and subpage bitmap.

This involves:

- Replace folio_clear_dirty_for_io() with folio_test_dirty()
  There is only one call site calling folio_clear_dirty_for_io() outside
  of subpage.c, that's inside extent_write_cache_pages() just before
  extent_writepage().

- Make btrfs_invalidate_folio() clear dirty range for the whole folio
  The function btrfs_invalidate_folio() is also called during
  extent_writepage().

  If we had a folio completely beyond isize, we call
  folio_invalidate() -> btrfs_invalidate_folio() to free the folio.

  Since we no longer have folio_clear_dirty_for_io() to clear the folio
  dirty flag, we must manually clear the folio dirty flag for the
  to-be-invalidated folio, and also clear the PAGECACHE_TAG_DIRTY tag.

  The tag clearing is done using a new helper,
  btrfs_clear_folio_dirty_tag(), which is almost the same as the old
  btree_clear_folio_dirty_tag(), but with minor improvements including:

  * Remove the folio_test_dirty() check
    We have already done an ASSERT().

  * Add an ASSERT() to make sure folio is mapped

- Add extra ASSERT()s before clearing folio private
  During development I hit dirty folios without the private flag set,
  and that caused a lot of ASSERT()s.
  The reason is that btrfs_invalidate_folio() is relying on the dirty
  flag being cleared when it's called from extent_writepage().

  Add extra ASSERT()s inside clear_folio_extent_mapped() to catch
  wild dirty/writeback flags.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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