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>