perf: Timehist account sch delay for scheduled out running

When using perf timehist, sch delay is only computed for a waking task,
not for a pre empted task. This patches changes sch delay to account for
both. This makes sense as testing scheduling policy need to consider the
effect of scheduling delay globally, not only for waking tasks.

Example of `perf timehist` report before the patch for `stress` task
competing with each other.

First column is wait time, second column sch delay, third column
runtime.

1.492060 [0000]  s    stress[81]                          1.999      0.000      2.000      R  next: stress[83]
1.494060 [0000]  s    stress[83]                          2.000      0.000      2.000      R  next: stress[81]
1.496060 [0000]  s    stress[81]                          2.000      0.000      2.000      R  next: stress[83]
1.498060 [0000]  s    stress[83]                          2.000      0.000      1.999      R  next: stress[81]

After the patch, it looks like this (note that all wait time is not zero
anymore):

1.492060 [0000]  s    stress[81]                          1.999      1.999      2.000      R  next: stress[83]
1.494060 [0000]  s    stress[83]                          2.000      2.000      2.000      R  next: stress[81]
1.496060 [0000]  s    stress[81]                          2.000      2.000      2.000      R  next: stress[83]
1.498060 [0000]  s    stress[83]                          2.000      2.000      1.999      R  next: stress[81]

Signed-off-by: Fernand Sieber <sieberf@amazon.com>
Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240618090339.87482-1-sieberf@amazon.com