gfs2: handle NULL rgd in set_rgrp_preferences

The function set_rgrp_preferences() does not handle the (rarely
returned) NULL value from gfs2_rgrpd_get_next() and this patch
fixes that.

The fs image in question is only 150MB in size which allows for
only 1 rgrp to be created. The in-memory rb tree has only 1 node
and when gfs2_rgrpd_get_next() is called on this sole rgrp, it
returns NULL. (Default behavior is to wrap around the rb tree and
return the first node to give the illusion of a circular linked
list. In the case of only 1 rgrp, we can't have
gfs2_rgrpd_get_next() return the same rgrp (first, last, next all
point to the same rgrp)... that would cause unintended consequences
and infinite loops.)
Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/rgrp.c

@@ -978,10 +978,10 @@ static void set_rgrp_preferences(struct gfs2_sbd *sdp)
 		rgd->rd_flags |= GFS2_RDF_PREFERRED;
 		for (i = 0; i < sdp->sd_journals; i++) {
 			rgd = gfs2_rgrpd_get_next(rgd);
-			if (rgd == first)
+			if (!rgd || rgd == first)
 				break;
 		}
-	} while (rgd != first);
+	} while (rgd && rgd != first);
 }
 
 /**