net: fix build erros with CONFIG_BUG=n, CONFIG_GENERIC_BUG=n

Fixed build errors introduced by commit 7ad6848c (ip: fix mc_loop
checks for tunnels with multicast outer addresses)
Signed-off-by: NOctavian Purdila <opurdila@ixiacom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -338,7 +338,7 @@ static inline int sk_mc_loop(struct sock *sk)
 		return inet6_sk(sk)->mc_loop;
 #endif
 	}
-	__WARN();
+	WARN_ON(1);
 	return 1;
 }