xfs: update realtime super every time we update the primary fs super

xfs: update realtime super every time we update the primary fs super

Source kernel commit: 76d3be00df91a56f7c05142ed500f8f8544d5457

Every time we update parts of the primary filesystem superblock that are
echoed in the rt superblock, we must update the rt super.  Avoid
changing the log to support logging to the rt device by using ordered
buffers.

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