net: tcp2: Implement net_tcp_input()

struct net_tcp_hdr *net_tcp_input(struct net_pkt *pkt,

				  struct net_pkt_data_access *tcp_access)

{

	ARG_UNUSED(pkt);

	ARG_UNUSED(tcp_access);

	struct net_tcp_hdr *tcp_hdr;

	if (IS_ENABLED(CONFIG_NET_TCP_CHECKSUM) &&

			net_if_need_calc_rx_checksum(net_pkt_iface(pkt)) &&

			net_calc_chksum_tcp(pkt) != 0U) {

		NET_DBG("DROP: checksum mismatch");

		goto drop;

	}

	tcp_hdr = (struct net_tcp_hdr *)net_pkt_get_data(pkt, tcp_access);

	if (tcp_hdr && !net_pkt_set_data(pkt, tcp_access)) {

		return tcp_hdr;

	}

drop:

	net_stats_update_tcp_seg_chkerr(net_pkt_iface(pkt));

	return NULL;

}