xfs: fix fork selection in xfs_find_trim_cow_extent

We should want to write directly into the data fork for blocks that don't
have an extent in the COW fork covering them yet.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>