bpf: Add BPF_SOCK_OPS_TSTAMP_SENDMSG_CB callback

This patch introduces a new callback in tcp_tx_timestamp() to correlate
tcp_sendmsg timestamp with timestamps from other tx timestamping
callbacks (e.g., SND/SW/ACK).

Without this patch, BPF program wouldn't know which timestamps belong
to which flow because of no socket lock protection. This new callback
is inserted in tcp_tx_timestamp() to address this issue because
tcp_tx_timestamp() still owns the same socket lock with
tcp_sendmsg_locked() in the meanwhile tcp_tx_timestamp() initializes
the timestamping related fields for the skb, especially tskey. The
tskey is the bridge to do the correlation.

For TCP, BPF program hooks the beginning of tcp_sendmsg_locked() and
then stores the sendmsg timestamp at the bpf_sk_storage, correlating
this timestamp with its tskey that are later used in other sending
timestamping callbacks.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250220072940.99994-11-kerneljasonxing@gmail.com

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