ipv4: Retire UDP-Lite.

ipv4: Retire UDP-Lite.

We have deprecated IPv6 UDP-Lite sockets.

Let's drop support for IPv4 UDP-Lite sockets as well.

Most of the changes are similar to the IPv6 patch: removing
udplite.c and udp_impl.h, marking most functions in udp_impl.h
as static, moving the prototype for udp_recvmsg() to udp.h, and
adding INDIRECT_CALLABLE_SCOPE for it.

In addition, the INET_DIAG support for UDP-Lite is dropped.

We will remove the remaining dead code in the following patches.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260311052020.1213705-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>