perf tools: Use scandir() to iterate threads when synthesizing PERF_RECORD_ events

Like in __event__synthesize_thread(), I think it's better to use
scandir() instead of the readdir() loop.  In case some malicious task
continues to create new threads, the readdir() loop will run over and
over to collect tids.  The scandir() also has the problem but the window
is much smaller since it doesn't do much work during the iteration.

Also add filter_task() function as we only care the tasks.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20210202090118.2008551-4-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>