x25: convert to getsockopt_iter

Convert X.25 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 put_user()/copy_to_user()
- Add linux/uio.h for copy_to_iter()

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260507-getsock_two-v2-3-5873111d9c12@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>