xfs: check the realtime superblock at mount time

xfs: check the realtime superblock at mount time

Check the realtime superblock at mount time, to ensure that the label
and uuids actually match the primary superblock on the data device.  If
the rt superblock is good, attach it to the xfs_mount so that the log
can use ordered buffers to keep this primary in sync with the primary
super on the data device.

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