sched/events: Introduce cfs_rq load tracking trace event

The trace event keys load and util (utilization) are mapped to:

 (1) load : cfs_rq->runnable_load_avg

 (2) util : cfs_rq->avg.util_avg

To let this trace event work for configurations w/ and w/o group
scheduling support for cfs (CONFIG_FAIR_GROUP_SCHED) the following
special handling is necessary for non-existent key=value pairs:

 path = "(null)" : In case of !CONFIG_FAIR_GROUP_SCHED.

 id   = -1       : In case of !CONFIG_FAIR_GROUP_SCHED.

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

 (1) a root task_group:

     cpu=4 path=/ id=1 load=6 util=331

 (2) a task_group:

     cpu=1 path=/tg1/tg11/tg111 id=4 load=538 util=522

 (3) an autogroup:

     cpu=3 path=/autogroup-18 id=0 load=997 util=517

 (4) w/o CONFIG_FAIR_GROUP_SCHED:

     cpu=0 path=(null) id=-1 load=314 util=289

The trace event is only defined for CONFIG_SMP.

The helper function __trace_sched_path() can be used to get the length
parameter of the dynamic array (path == NULL) and to copy the path into
it (path != NULL).

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