perf sched timehist: Split is_idle_sample()

The is_idle_sample() function actually does more than determining
whether sample come from idle task.  Split the callchain part into
save_task_callchain() to make it clearer.

Also checking prev_pid from trace data looks preferred than just
checking sample->pid since it's possible, although rare, to have invalid
0 pid/tid on scheduling an exiting task.

Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: David Ahern <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Remove some needless () in some return statements ]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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