xfs: metadata files can have xattrs if metadir is enabled

xfs: metadata files can have xattrs if metadir is enabled

If metadata directory trees are enabled, it's possible that some future
metadata file might want to store information in extended attributes.
Or, if parent pointers are enabled, then children of the metadir tree
need parent pointers.  Either way, we start allowing xattr data when
metadir is enabled, so we now need check and repair to examine attr
forks for metadata files on metadir filesystems.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>