rtmutex-tester: fix mode of sysfs files

This properly sets the modes of the sysfs files.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>