btrfs: use dirty flag to check if an ordered extent needs to be truncated

btrfs: use dirty flag to check if an ordered extent needs to be truncated

Currently there are only two folio ordered flag users:

- extent_writepage_io()
  To ensure the folio range has an ordered extent covering it.
  This is from the legacy COW fixup mechanism, which is already removed
  and only a simple check is left.

- btrfs_invalidate_folio()
  This is to avoid race with end_bbio_data_write(), where
  btrfs_finish_ordered_extent() will be called to handle the OE
  finishing.

But for btrfs_invalidate_folio() we have already waited for the folio
writeback to finish, and locked the folio.
This means we can use the dirty flag to check if a range is already
submitted or not.

If the OE range is not dirty, it means the range has been submitted and
its dirty flag was cleared. And since we have already waited for
writeback, the endio function will handle the OE finishing.
Thus if the range is not dirty, we must skip the range.

If the OE range is dirty, it means we have allocated an ordered extent but
have not yet submitted the range. And that's exactly the case where we need
to truncate the ordered extent.

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