GFS2: Fix one RG corner case

For filesystems with only a single resource group, we need to be careful
that the allocation loop will not land up with a NULL resource group. This
fixes a bug in a previous patch where the gfs2_rgrpd_get_next() function
was being used instead of gfs2_rgrpd_get_first()

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>