gfs2: check for empty rgrp tree in gfs2_ri_update

If gfs2 tries to mount a (corrupt) file system that has no resource
groups it still tries to set preferences on the first one, which causes
a kernel null pointer dereference. This patch adds a check to function
gfs2_ri_update so this condition is detected and reported back as an
error.

Reported-by: syzbot+e3f23ce40269a4c9053a@syzkaller.appspotmail.com
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/rgrp.c

@@ -985,6 +985,10 @@ static int gfs2_ri_update(struct gfs2_inode *ip)
 	if (error < 0)
 		return error;
 
+	if (RB_EMPTY_ROOT(&sdp->sd_rindex_tree)) {
+		fs_err(sdp, "no resource groups found in the file system.\n");
+		return -ENOENT;
+	}
 	set_rgrp_preferences(sdp);
 
 	sdp->sd_rindex_uptodate = 1;