prctl: make PR_SET_TIMERSLACK_PID pid namespace aware

Make PR_SET_TIMERSLACK_PID consider pid namespace and resolve the
target pid in the caller's namespace. Otherwise, calls from pid
namespace other than init would fail or affect the wrong task.

Change-Id: I1da15196abc4096536713ce03714e99d2e63820a
Signed-off-by: Micha Kalfon <micha@cellrox.com>
Acked-by: Oren Laadan <orenl@cellrox.com>