netlink: convert to getsockopt_iter

netlink: convert to getsockopt_iter

Convert AF_NETLINK's getsockopt implementation 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) and returned size (output)
- Use copy_to_iter() instead of put_user()/copy_to_user()
- For NETLINK_LIST_MEMBERSHIPS: walk the groups bitmap and emit each
  u32 sequentially via copy_to_iter(), then set opt->optlen to the
  total size required (ALIGN(BITS_TO_BYTES(ngroups), sizeof(u32))).
  The wrapper writes opt->optlen back to userspace even on partial
  failure, preserving the existing API that lets userspace discover
  the needed allocation size.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260501-getsock_one-v1-1-810ce23ea70e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>