trace: tcp: Add tracepoint for tcp_sendmsg_locked()

trace: tcp: Add tracepoint for tcp_sendmsg_locked()

Add a tracepoint to monitor TCP send operations, enabling detailed
visibility into TCP message transmission.

Create a new tracepoint within the tcp_sendmsg_locked function,
capturing traditional fields along with size_goal, which indicates the
optimal data size for a single TCP segment. Additionally, a reference to
the struct sock sk is passed, allowing direct access for BPF programs.
The implementation is largely based on David's patch[1] and suggestions.

Link: https://lore.kernel.org/all/70168c8f-bf52-4279-b4c4-be64527aa1ac@kernel.org/
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250408-tcpsendmsg-v3-2-208b87064c28@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>