sched/events: Introduce task_group load tracking trace event

The trace event key load is mapped to:

 (1) load : cfs_rq->tg->load_avg

The cfs_rq owned by the task_group is used as the only parameter for the
trace event because it has a reference to the taskgroup and the cpu.
Using the taskgroup as a parameter instead would require the cpu as a
second parameter. A task_group is global and not per-cpu data. The cpu
key only tells on which cpu the value was gathered.

The following list shows examples of the key=value pairs for:

 (1) a task group:

     cpu=1 path=/tg1/tg11/tg111 id=4 load=517

 (2) an autogroup:

     cpu=1 path=/autogroup-10 id=0 load=1050

We don't maintain a load signal for a root task group.

The trace event is only defined if cfs group scheduling support
(CONFIG_FAIR_GROUP_SCHED) is enabled.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>