tcp: reorder tcp_congestion_ops for better cache locality

Group all the often used fields in the first cache line,
to reduce cache line misses.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>