nsfs: fix ioctl declaration

The kernel is writing an object of type __u64, so the ioctl has to be
defined to _IOR(NSIO, 0x5, __u64) instead of _IO(NSIO, 0x5).

Reported-by: Dmitry V. Levin <ldv@strace.io>
Link: https://lore.kernel.org/r/20240730164554.GA18486@altlinux.org


Signed-off-by: Christian Brauner <brauner@kernel.org>