sched: Avoid scale real weight down to zero

During our testing, we found a case that shares no longer
working correctly, the cgroup topology is like:

  /sys/fs/cgroup/cpu/A		(shares=102400)
  /sys/fs/cgroup/cpu/A/B	(shares=2)
  /sys/fs/cgroup/cpu/A/B/C	(shares=1024)

  /sys/fs/cgroup/cpu/D		(shares=1024)
  /sys/fs/cgroup/cpu/D/E	(shares=1024)
  /sys/fs/cgroup/cpu/D/E/F	(shares=1024)

The same benchmark is running in group C & F, no other tasks are
running, the benchmark is capable to consumed all the CPUs.

We suppose the group C will win more CPU resources since it could
enjoy all the shares of group A, but it's F who wins much more.

The reason is because we have group B with shares as 2, since
A->cfs_rq.load.weight == B->se.load.weight == B->shares/nr_cpus,
so A->cfs_rq.load.weight become very small.

And in calc_group_shares() we calculate shares as:

  load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
  shares = (tg_shares * load) / tg_weight;

Since the 'cfs_rq->load.weight' is too small, the load become 0
after scale down, although 'tg_shares' is 102400, shares of the se
which stand for group A on root cfs_rq become 2.

While the se of D on root cfs_rq is far more bigger than 2, so it
wins the battle.

Thus when scale_load_down() scale real weight down to 0, it's no
longer telling the real story, the caller will have the wrong
information and the calculation will be buggy.

This patch add check in scale_load_down(), so the real weight will
be >= MIN_SHARES after scale, after applied the group C wins as
expected.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/38e8e212-59a1-64b2-b247-b6d0b52d8dc1@linux.alibaba.com