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: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ip.h b/include/net/ip.h
index d9a0e74..fb63371 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -338,7 +338,7 @@
 		return inet6_sk(sk)->mc_loop;
 #endif
 	}
-	__WARN();
+	WARN_ON(1);
 	return 1;
 }