selftests/landlock: Check IOCTL restrictions for named UNIX domain sockets

The LANDLOCK_ACCESS_FS_IOCTL_DEV right should have no effect on the use of
named UNIX domain sockets.

Suggested-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240419161122.2023765-7-gnoack@google.com


[mic: Add missing stddef.h for offsetof()]
Signed-off-by: Mickaël Salaün <mic@digikod.net>