af_packet: convert to getsockopt_iter

af_packet: convert to getsockopt_iter

Convert AF_PACKET'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 PACKET_HDRLEN which reads from optval: use opt->iter_in with
  copy_from_iter() for the input read, then the common opt->iter_out
  copy_to_iter() epilogue handles the output

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260408-getsockopt-v3-3-061bb9cb355d@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>