nsfs: validate ioctls

Nsfs supports extensible and non-extensible ioctls. Validate both types
to prevent confusion.

Link: https://lore.kernel.org/r/20250219-work-nsfs-v1-1-21128d73c5e8@kernel.org


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