sb_rgcount for file system without a RT subvolume and without the metadir/rtgroup feature should be 1. That is because we have an implicit default rtgroup that points to the global bitmap and summary inodes, which exist even with zero rtblocks. Now for a kernel without CONFIG_XFS_RT that probably does not matter, but I'd prefer to keep the value consistent for CONFIG_XFS_RT vs !CONFIG_XFS_RT.
