ocfs2: 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>

fs/ocfs2/dcache.c

@@ -56,7 +56,7 @@ static int ocfs2_dentry_revalidate(struct dentry *dentry,
 	int ret = 0;    /* if all else fails, just return false */
 	struct ocfs2_super *osb;
 
-	if (nd->flags & LOOKUP_RCU)
+	if (nd && nd->flags & LOOKUP_RCU)
 		return -ECHILD;
 
 	inode = dentry->d_inode;