selftests/nsfs: add ioctl validation tests

Add simple tests to validate that non-nsfs ioctls are rejected.

Link: https://lore.kernel.org/r/20250219-work-nsfs-v1-2-21128d73c5e8@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>