perf tools: Fix detection of stack area

perf tools: Fix detection of stack area

Output of /proc/<pid>/maps contains helpful information to anonymous
mappings like stack, heap, ...  For the case of stack, it can show
multiple stack area for each thread in the process:

  $ cat /proc/$(pidof gnome-shell)/maps | grep stack
  7fe019946000-7fe01a146000 rw-p 00000000 00:00 0          [stack:1624]
  7fe040e32000-7fe041632000 rw-p 00000000 00:00 0          [stack:1451]
  7fe041643000-7fe041e43000 rw-p 00000000 00:00 0          [stack:1450]
  7fe04204b000-7fe04284b000 rw-p 00000000 00:00 0          [stack:1449]
  7fe042a7e000-7fe04327e000 rw-p 00000000 00:00 0          [stack:1446]
  7fe0432ff000-7fe043aff000 rw-p 00000000 00:00 0          [stack:1445]
  7fe043b00000-7fe044300000 rw-p 00000000 00:00 0          [stack:1444]
  7fe044301000-7fe044b01000 rw-p 00000000 00:00 0          [stack:1443]
  7fe044b02000-7fe045302000 rw-p 00000000 00:00 0          [stack:1442]
  7fe045303000-7fe045b03000 rw-p 00000000 00:00 0          [stack:1441]
  7fe045b04000-7fe046304000 rw-p 00000000 00:00 0          [stack:1440]
  7fe046305000-7fe046b05000 rw-p 00000000 00:00 0          [stack:1439]
  7fe046b06000-7fe047306000 rw-p 00000000 00:00 0          [stack:1438]
  7fff4b16f000-7fff4b190000 rw-p 00000000 00:00 0          [stack]

However perf only knew about the main thread's.  Fix it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1352273234-28912-4-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>