selftests: net: getsockopt_iter: cleanup

selftests: net: getsockopt_iter: cleanup

Apply two cleanups suggested by Stanislav and bobby on the original
selftest series:

- Reorder local variable declarations into reverse christmas-tree
  order (longest line first). Because that ordering puts socklen_t
  optlen before the variable whose size it stores, the
  "optlen = sizeof(...)" initializer is moved out of the declaration
  to a plain assignment in the test body, as Stanislav suggested.

- Add ASSERT_EQ(optlen, ...) on every error path so the value the
  kernel writes back to the userspace optlen is pinned down even
  when the syscall returns -1. With do_sock_getsockopt() now writing
  opt->optlen back to userspace unconditionally, asserting that the
  netlink/vsock error paths leave the original input length untouched
  guards against future regressions.

Bobby Eshleman pointed out that
SO_VM_SOCKETS_CONNECT_TIMEOUT_NEW/OLD return a sock_timeval-shaped
payload (16 bytes on 64-bit), which is wider than the u64 case
already covered. Add four tests that exercise this path:

- connect_timeout_new_exact         exact-size buffer
- connect_timeout_new_oversize_clamped  oversize buffer, clamped
- connect_timeout_new_undersize     undersize -> -EINVAL, optlen
                                    untouched
- connect_timeout_old_exact         exact-size buffer for OLD optname

Suggested-by: Stanislav Fomichev <sdf@fomichev.me>
Suggested-by: Bobby Eshleman <bobbyeshleman@meta.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260507-getsock_two-v2-5-5873111d9c12@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>