drivers/regulator/virtual: avoid world-writable sysfs files.

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

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc drivers/regulator/virtual.c
Simple merge