psi: Reduce calls to sched_clock() in psi

We noticed that the cost of psi increases with the increase in the
levels of the cgroups. Particularly the cost of cpu_clock() sticks out
as the kernel calls it multiple times as it traverses up the cgroup
tree. This patch reduces the calls to cpu_clock().

Performed perf bench on Intel Broadwell with 3 levels of cgroup.

Before the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.747 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.516 [sec]

       3.516689 usecs/op
         284358 ops/sec

After the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.640 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.329 [sec]

       3.329820 usecs/op
         300316 ops/sec

Signed-off-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20210321205156.4186483-1-shakeelb@google.com