sched/fair: Make sure that /proc/sched_debug shows at least the root cfs_rqs

Commit 039ae8bc ("sched/fair: Fix O(nr_cgroups) in the load
balancing path") removes the root cfs_rqs from rq->leaf_cfs_rq_list in
case its PELT values are completely decayed.

This leads to /proc/sched_debug not showing the debug values of these
cfs_rqs although those of the rt_rqs and dl_rgs are listed. Moreover,
the per-cfs_rq debug values contain non-PELT data as well.

There are two possible ways to fix this:

(1) Keep the root cfs_rq's in the list.

(2) Print the root cfs_rq in print_cfs_stats() and then iterate over
    the list while ignoring the root cfs_rq.

The former is implemented here.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>