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

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

Every time we update parts of the primary filesystem superblock that are
echoed in the primary rt super, we should update that primary realtime
super.  Avoid an ondisk log format change by using ordered buffers to
write the primary rt super.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>