perf thread-stack: Simplify some code in thread_stack__process()

In preparation for fixing thread stack processing for the idle task,
simplify some code in thread_stack__process().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20181221120620.9659-2-adrian.hunter@intel.com


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