schied/fair: Skip calculating @contrib without load

Because of the:

	if (!load)
		runnable = running = 0;

clause in ___update_load_sum(), all the actual users of @contrib in
accumulate_sum():

	if (load)
		sa->load_sum += load * contrib;
	if (runnable)
		sa->runnable_load_sum += runnable * contrib;
	if (running)
		sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;

don't happen, and therefore we don't care what @contrib actually is and
calculating it is pointless.

If we count the times when @load equals zero and not as below:

	if (load) {
		load_is_not_zero_count++;
		contrib = __accumulate_pelt_segments(periods,
				1024 - sa->period_contrib,delta);
	} else
		load_is_zero_count++;

As we can see, load_is_zero_count is much bigger than
load_is_zero_count, and the gap is gradually widening:

	load_is_zero_count:            6016044 times
	load_is_not_zero_count:         244316 times
	19:50:43 up 1 min,  1 user,  load average: 0.09, 0.06, 0.02

	load_is_zero_count:            7956168 times
	load_is_not_zero_count:         261472 times
	19:51:42 up 2 min,  1 user,  load average: 0.03, 0.05, 0.01

	load_is_zero_count:           10199896 times
	load_is_not_zero_count:         278364 times
	19:52:51 up 3 min,  1 user,  load average: 0.06, 0.05, 0.01

	load_is_zero_count:           14333700 times
	load_is_not_zero_count:         318424 times
	19:54:53 up 5 min,  1 user,  load average: 0.01, 0.03, 0.00

Perhaps we can gain some performance advantage by saving these
unnecessary calculation.

Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot &lt; <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1576208740-35609-1-git-send-email-rocking@linux.alibaba.com