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>