tcp: Remove sk->sk_prot->orphan_count.

tcp: Remove sk->sk_prot->orphan_count.

TCP tracks the number of orphaned (SOCK_DEAD but not yet destructed)
sockets in tcp_orphan_count.

In some code that was shared with DCCP, tcp_orphan_count is referenced
via sk->sk_prot->orphan_count.

Let's reference tcp_orphan_count directly.

inet_csk_prepare_for_destroy_sock() is moved to inet_connection_sock.c
due to header dependency.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250829215641.711664-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>