net: call getsockopt_iter if available

net: call getsockopt_iter if available

Update do_sock_getsockopt() to use the new getsockopt_iter callback
when available. Add do_sock_getsockopt_iter() helper that:

1. Reads optlen from user/kernel space
2. Initializes a sockopt_t with the appropriate iov_iter (kvec for
   kernel, ubuf for user buffers) and sets opt.optlen
3. Calls the protocol's getsockopt_iter callback
4. Writes opt.optlen back to user/kernel space

The optlen is always written back, even on failure. Some protocols
(e.g. CAN raw) return -ERANGE and set optlen to the required buffer
size so userspace knows how much to allocate.

The callback is responsible for setting opt.optlen to indicate the
returned data size.

Important to say that  iov_out does not need to be copied back in
do_sock_getsockopt().

When optval is not kernel (the userspace path), sockptr_to_sockopt()
sets up opt->iter_out as a ITER_DEST ubuf iterator pointing directly at
the userspace buffer (optval.user). So when getsockopt_iter
implementations call copy_to_iter(..., &opt->iter_out), the data is
written directly to userspace — no intermediate kernel buffer is
involved.

When optval.is_kernel is true (the in-kernel path, e.g. from io_uring),
the kvec points at the already-provided kernel buffer (optval.kernel),
so the data lands in the caller's buffer directly via the kvec-backed
iterator.

In both cases the iterator writes to the final destination in-place at
protocol callback. There's nothing to copy back — only optlen needs to
be written back.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260408-getsockopt-v3-2-061bb9cb355d@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>