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>

fs/gfs2/rgrp.c

@@ -1776,10 +1776,11 @@ static u32 gfs2_orlov_skip(const struct gfs2_inode *ip)
 static bool gfs2_select_rgrp(struct gfs2_rgrpd **pos, const struct gfs2_rgrpd *begin)
 {
 	struct gfs2_rgrpd *rgd = *pos;
+	struct gfs2_sbd *sdp = rgd->rd_sbd;
 
 	rgd = gfs2_rgrpd_get_next(rgd);
 	if (rgd == NULL)
-		rgd = gfs2_rgrpd_get_next(NULL);
+		rgd = gfs2_rgrpd_get_first(sdp);
 	*pos = rgd;
 	if (rgd != begin) /* If we didn't wrap */
 		return true;