xfs: only flush when COW fork blocks overlap data fork holes

xfs: only flush when COW fork blocks overlap data fork holes

The zero range hole mapping flush case has been lifted from iomap
into XFS. Now that we have more mapping context available from the
->iomap_begin() handler, we can isolate the flush further to when we
know a hole is fronted by COW blocks.

Rather than purely rely on pagecache dirty state, explicitly check
for the case where a range is a hole in both forks. Otherwise trim
to the range where there does happen to be overlap and use that for
the pagecache writeback check. This might prevent some spurious
zeroing, but more importantly makes it easier to remove the flush
entirely.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>