tcp: Move TCP-specific diag functions to tcp_diag.c.

tcp: Move TCP-specific diag functions to tcp_diag.c.

tcp_diag_dump() / tcp_diag_dump_one() is just a wrapper of
inet_diag_dump_icsk() / inet_diag_dump_one_icsk(), respectively.

Let's inline them in tcp_diag.c and move static callees as well.

Note that inet_sk_attr_size() is merged into tcp_diag_get_aux_size(),
and we remove inet_diag_handler.idiag_get_aux_size() accordingly.

While at it, BUG_ON() is replaced with DEBUG_NET_WARN_ON_ONCE().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250822190803.540788-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>