udp: move udp6_csum_init() back to net/ipv6/udp.c

This function has a single caller in net/ipv6/udp.c.

Move it there so that the compiler can decide to (auto)inline
it if he prefers to. IBT glue is removed anyway.

With clang, we can see it was able to inline it and also
inlined one other helper at the same time.

UDPLITE removal will also help.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 1/0 up/down: 840/-785 (55)
Function                                     old     new   delta
__udp6_lib_rcv                              1247    2087    +840
__pfx_udp6_csum_init                          16       -     -16
udp6_csum_init                               769       -    -769
Total: Before=25074399, After=25074454, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260223093445.3696368-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>