ANDROID: sched/walt: Fix missing locking in WALT port

commit 26c21548 ("ANDROID: sched: Introduce Window Assisted Load
Tracking (WALT)") which was a forward port of WALT from android 4.4
missed locking for 2 set_task_cpu() calls, re-added here.

Signed-off-by: Andres Oportus <andresoportus@google.com>