xfs: don't return an error from xfs_update_last_rtgroup_size for !XFS_RT

Non-rtg file systems have a fake RT group even if they do not have a RT
device, and thus an rgcount of 1.  Ensure xfs_update_last_rtgroup_size
doesn't fail when called for !XFS_RT to handle this case.

Fixes: 87fe4c34 ("xfs: create incore realtime group structures")
Reported-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>