sched/fair: Add cgroup_mode: concur

Improve upon the previous scheme ("max") by no longer assuming maximal
concurrency. Instead scale by: 'min(nr_tasks, nr_cpus)'. This handles
the low concurrency cases more gracefully:

	F_g_n' = min(M, N) * F_g_n

Notably this is the first mode where:

	avg(F_g_n) = 1

In the single task case it reduces to ("smp") and then it nicely scales up
until it hits N, where it behaves like ("max").

This is no longer clipped at nice -20. Strictly speaking it isn't different
from the normal SMP scenario where all tasks are extremely unbalanced. There
are no unnatural inflations in this scheme.

The meaning of "cpu.weight" would be: weight per active CPU.

NOTE: Compute the group wide number of tasks by extending the tg->load_avg
computation with tg->runnable_avg, since cfs_rq->runnable_avg is based on
cfs_rq->h_nr_running.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260605124051.740585993%40infradead.org

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