tcp: rstreason: introduce SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT for active reset

Introducing this to show the users the reason of keepalive timeout.

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