gfs2: fix d_revalidate oopsen on NFS exports

can't blindly check nd->flags in ->d_revalidate()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/gfs2/dentry.c b/fs/gfs2/dentry.c
index 4a45633..0da8da2 100644
--- a/fs/gfs2/dentry.c
+++ b/fs/gfs2/dentry.c
@@ -44,7 +44,7 @@
 	int error;
 	int had_lock = 0;
 
-	if (nd->flags & LOOKUP_RCU)
+	if (nd && nd->flags & LOOKUP_RCU)
 		return -ECHILD;
 
 	parent = dget_parent(dentry);