We first locate the (local port) hash chain head
If few sockets are in this chain, we proceed with previous lookup algo.

If too many sockets are listed, we take a look at the secondary
(port, address) hash chain.

We choose the shortest chain and proceed with a RCU lookup on the elected chain.

But, if we chose (port, address) chain, and fail to find a socket on given address,
 we must try another lookup on (port, in6addr_any) chain to find sockets not bound
to a particular IP.

-> No extra cost for typical setups, where the first lookup will probabbly
be performed.

RCU lookups everywhere, we dont acquire spinlock.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>