ipv6: Retire UDP-Lite.

ipv6: Retire UDP-Lite.

As announced in commit be28c14ac8bb ("udplite: Print deprecation
notice."), it's time to deprecate UDP-Lite.

As a first step, let's drop support for IPv6 UDP-Lite sockets.

We will remove the remaining dead code gradually.

Along with the removal of udplite.c, most of the functions exposed
via udp_impl.h are made static.

The prototypes of udpv6_sendmsg() and udpv6_recvmsg() are moved
to udp.h, but only udpv6_recvmsg() has INDIRECT_CALLABLE_DECLARE()
because udpv6_sendmsg() is exported for rxrpc since commit ed472b0c8783
("rxrpc: Call udp_sendmsg() directly").

Also, udpv6_recvmsg() needs INDIRECT_CALLABLE_SCOPE for
CONFIG_MITIGATION_RETPOLINE=n.

Note that udplite.h is included temporarily for udplite_csum().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260311052020.1213705-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>