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 @@ void __skb_get_rxhash(struct sk_buff *skb)
  }
  }
  break;
+ case IPPROTO_IPIP:
+ goto again;
  default:
  break;
  }