rps: support IPIP encapsulation

Skip IPIP header to get proper layer-4 information.

Like GRE tunnels, this only works if rxhash is not already provided by
the device itself (ethtool -K ethX rxhash off), to allow kernel compute
a software rxhash.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2608,6 +2608,8 @@ again:
 			}
 		}
 		break;
+	case IPPROTO_IPIP:
+		goto again;
 	default:
 		break;
 	}