net: introduce sk_ehashfn() helper

Goal is to unify IPv4/IPv6 inet_hash handling, and use common helpers
for all kind of sockets (full sockets, timewait and request sockets)

inet_sk_ehashfn() becomes sk_ehashfn() but still only copes with IPv4

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>