GFS2: kernel panic with small gfs2 filesystems - 1 RG

In the unlikely setup where there's only one resource group in the gfs2
filesystem, gfs2_rgrpd_get_next() returns a NULL rgd that is not dealt with
properly, causing a kernel NULL ptr dereference. This patch fixes this issue.
Signed-off-by: NAbhi Das <adas@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/gfs2/rgrp.c

@@ -1276,6 +1276,7 @@ int gfs2_inplace_reserve(struct gfs2_inode *ip, u32 requested)
 			/* fall through */
 		case GLR_TRYFAILED:
 			rgd = gfs2_rgrpd_get_next(rgd);
+			rgd = rgd ? : begin; /* if NULL, wrap */
 			if (rgd != begin) /* If we didn't wrap */
 				break;