xfs: check the metadata directory inumber in superblocks

When metadata directories are enabled, make sure that the secondary
superblocks point to the metadata directory.  This isn't strictly
required because the secondaries are only used to recover damaged
filesystems, and the metadir root inumber is fixed.

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