tcp: accecn: add AccECN rx byte counters

tcp: accecn: add AccECN rx byte counters

These three byte counters track IP ECN field payload byte sums for
all arriving (acceptable) packets for ECT0, ECT1, and CE. The
AccECN option (added by a later patch in the series) echoes these
counters back to sender side; therefore, it is placed within the
group of tcp_sock_write_txrx.

Below are the pahole outcomes before and after this patch, in which
the group size of tcp_sock_write_txrx is increased from 95 + 4 to
107 + 4 and an extra 4-byte hole is created but will be exploited
in later patches:

[BEFORE THIS PATCH]
struct tcp_sock {
    [...]
    u32                        delivered_ce;         /*  2576     4 */
    u32                        received_ce;          /*  2580     4 */
    u32                        app_limited;          /*  2584     4 */
    u32                        rcv_wnd;              /*  2588     4 */
    struct tcp_options_received rx_opt;              /*  2592    24 */
    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2616     0 */

    [...]
    /* size: 3200, cachelines: 50, members: 166 */
}

[AFTER THIS PATCH]
struct tcp_sock {
    [...]
    u32                        delivered_ce;         /*  2576     4 */
    u32                        received_ce;          /*  2580     4 */
    u32                        received_ecn_bytes[3];/*  2584    12 */
    u32                        app_limited;          /*  2596     4 */
    u32                        rcv_wnd;              /*  2600     4 */
    struct tcp_options_received rx_opt;              /*  2604    24 */
    __cacheline_group_end__tcp_sock_write_txrx[0];   /*  2628     0 */
    /* XXX 4 bytes hole, try to pack */

    [...]
    /* size: 3200, cachelines: 50, members: 167 */
}

Signed-off-by: Ilpo Järvinen <ij@kernel.org>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Co-developed-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916082434.100722-4-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>