tcp: Maintain dynamic metrics in local cache.
Maintain a local hash table of TCP dynamic metrics blobs. Computed TCP metrics are no longer maintained in the route metrics. The table uses RCU and an extremely simple hash so that it has low latency and low overhead. A simple hash is legitimate because we only make metrics blobs for fully established connections. Some tweaking of the default hash table sizes, metric timeouts, and the hash chain length limit certainly could use some tweaking. But the basic design seems sound. With help from Eric Dumazet and Joe Perches. Signed-off-by: David S. Miller <email@example.com>