fix sys_getppid oopses on debug kernel

sys_getppid() optimization can access a freed memory.  On kernels with
DEBUG_SLAB turned ON, this results in Oops.  As Dave Hansen noted, this
optimization is also unsafe for memory hotplug.

So this patch always takes the lock to be safe.

Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>