atm: convert to getsockopt_iter

atm: convert to getsockopt_iter

Convert the ATM SVC and PVC sockets, along with the shared
vcc_getsockopt() helper, to use the new getsockopt_iter callback with
sockopt_t.

Key changes:
- Replace (char __user *optval, int __user *optlen) with sockopt_t *opt
- Use opt->optlen for buffer length (input)
- Use copy_to_iter() instead of put_user()/copy_to_user()

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260520-getsock_four-v3-2-b8c0b16b7780@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>