can: raw: convert to getsockopt_iter

can: raw: convert to getsockopt_iter

Convert CAN raw socket'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 copy_to_user()
- For CAN_RAW_FILTER and CAN_RAW_XL_VCID_OPTS: on -ERANGE, set
  opt->optlen to the required buffer size. The wrapper writes this
  back to userspace even on error, 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/20260408-getsockopt-v3-4-061bb9cb355d@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>