sched/fair: Change the default cgroup_mode to concur

For all the reasons described in the preceding patches, the way cgroup
weight is computed is problematic. However, changing it is bound to
also lead to trouble. Esp. since people might have taken to inflating
the weight value where they can.

Since things are configurable, change the default and hope this serves
more people than it hurts, esp. in the longer run.

Specifically, this prepares for a flattened runqueue, where the hierarchical
weight becomes far more important (F_g^d terms), so getting rid of small F_g is
imperative.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>