net: Add functions to get skb->hash based on flow structures

Add skb_get_hash_flowi6 and skb_get_hash_flowi4 which derive an sk_buff
hash from flowi6 and flowi4 structures respectively. These functions
can be called when creating a packet in the output path where the new
sk_buff does not yet contain a fully formed packet that is parsable by
flow dissector.

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

diff --cc include/linux/skbuff.h
Simple merge

diff --cc net/core/flow_dissector.c
Simple merge