In line with practice for module parameters, we're adding a build-time
check that sysfs files aren't world-writable.

Cc: Christopher Brannon <chris@the-brannons.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>