sysfs: disallow world-writable files.

This check was introduced in 2006 by Alexey Dobriyan (9774a1f5

)
for module parameters; we removed it when we unified the check into
VERIFY_OCTAL_PERMISSIONS() as sysfs didn't have the same requirement.
Now all those users are fixed, reintroduce it.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>