xfs: metadata files can have xattrs if metadir is enabled

If parent pointers are enabled, then metadata files will store parent
pointers in xattrs, just like files in the user visible directory tree.
Therefore, scrub and repair need to handle attr forks for metadata files
on metadir filesystems.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>