net: Add function to set the rxhash

The function skb_set_rxash was added for drivers to call to set
the rxhash in an skb. The type of hash is also specified as
a parameter (L2, L3, L4, or unknown type).

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>