xfs: refactor directory tree root predicates

Metadata directory trees make reasoning about the parent of a file more
difficult.  Traditionally, user files are children of sb_rootino, and
metadata files are "children" of the superblock.  Now, we add a third
possibility -- some metadata files can be children of sb_metadirino, but
the classic ones (rt free space data and quotas) are left alone.

Let's add some helper functions (instead of open-coding the logic
everywhere) to make scrub logic easier to understand.

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