inet: switch IP ID generator to siphash

inet: switch IP ID generator to siphash

According to Amit Klein and Benny Pinkas, IP ID generation is too weak
and might be used by attackers.

Even with recent net_hash_mix() fix (netns: provide pure entropy for net_hash_mix())
having 64bit key and Jenkins hash is risky.

It is time to switch to siphash and its 128bit keys.

Signed-off-by: Eric Dumazet <[email protected]>
Reported-by: Amit Klein <[email protected]>
Reported-by: Benny Pinkas <[email protected]>
Signed-off-by: David S. Miller <[email protected]>