When we call growfs on the data device, we update the secondary
superblocks to reflect the updated filesystem geometry.  We need to do
this for growfs on the realtime volume too, because a future xfs_repair
run could try to fix the filesystem using a backup superblock.

This was observed by the online superblock scrubbers while running
xfs/233.  One can also trigger this by growing an rt volume, cycling the
mount, and creating new rt files.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>