perf thread-stack: Allocate an array of thread stacks

In preparation for fixing thread stack processing for the idle task,
allocate an array of thread stacks.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20181221120620.9659-7-adrian.hunter@intel.com


[ No need to check for NULL when calling zfree(), noticed by Jiri Olsa ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>