tcp: add cwnd_event_tx_start to tcp_congestion_ops

tcp: add cwnd_event_tx_start to tcp_congestion_ops

(tcp_congestion_ops)->cwnd_event() is called very often, with
@event oscillating between CA_EVENT_TX_START and other values.

This is not branch prediction friendly.

Provide a new cwnd_event_tx_start pointer dedicated for CA_EVENT_TX_START.

Both BBR and CUBIC benefit from this change, since they only care
about CA_EVENT_TX_START.

No change in kernel size:

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux
add/remove: 4/4 grow/shrink: 3/1 up/down: 564/-568 (-4)
Function                                     old     new   delta
bbr_cwnd_event_tx_start                        -     450    +450
cubictcp_cwnd_event_tx_start                   -      70     +70
__pfx_cubictcp_cwnd_event_tx_start             -      16     +16
__pfx_bbr_cwnd_event_tx_start                  -      16     +16
tcp_unregister_congestion_control             93      99      +6
tcp_update_congestion_control                518     521      +3
tcp_register_congestion_control              422     425      +3
__tcp_transmit_skb                          3308    3306      -2
__pfx_cubictcp_cwnd_event                     16       -     -16
__pfx_bbr_cwnd_event                          16       -     -16
cubictcp_cwnd_event                           80       -     -80
bbr_cwnd_event                               454       -    -454
Total: Before=25240512, After=25240508, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260323234920.1097858-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>