Merge branch 'net-convert-four-more-protocols-to-getsockopt_iter'

Breno Leitao says:

====================
net: convert four more protocols to getsockopt_iter

Continue the work to convert protocols to the new getsockopt_iter API.

Convert four additional getsockopt implementations to the new
sockopt_t/getsockopt_iter callback:

  - MCTP
  - LLC
  - X.25
  - KCM

These are mechanical, ABI-preserving conversions following the same
pattern as the previously converted protocols (af_packet, can/raw,
af_netlink, af_vsock): the (char __user *optval, int __user *optlen)
pair is replaced with a single sockopt_t *opt that carries the buffer
length on input and the returned size on output, and exposes an iov_iter
for the copy-out path. put_user()/copy_to_user() pairs are replaced with
a single copy_to_iter() per option, and the wrapper in
do_sock_getsockopt() handles writing optlen back to userspace.

I picked these four because each is small and self-contained.
====================

Link: https://patch.msgid.link/20260507-getsock_two-v2-0-5873111d9c12@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>