[SCTP] Use put_user() in get_peer_addr_params (reported by yjf@standford.edu)

Standford Checker reported direct touch of user space. 

net/sctp/socket.c

@@ -2279,7 +2279,9 @@ static int sctp_getsockopt_peer_addr_params(struct sock *sk, int len,
 
 	if (copy_to_user(optval, &params, len))
 		return -EFAULT;
-	*optlen = len;
+
+	if (put_user(len, optlen))
+		return -EFAULT;
 
 	return 0;
 }