xfs: use metadir for quota inodes

Store the quota inodes in the /quota metadata directory if metadir is
enabled.  This enables us to stop using the sb_[ugp]uotino fields in the
superblock.  From this point on, all metadata files will be children of
the metadata directory tree root.

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