tcp: sizeof struct tcp_skb_cb is 44

Correct comment stating sizeof(struct tcp_skb_cb) is 36 or 40, since its
44 bytes, since commit 951dbc8a ([IPV6]: Move nextheader offset
to the IP6CB).

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>