perf record: Fix existing process callgraph symbol

When 'perf record -g' a existing process, even with debuginfo
packages, still cannnot get symbol from 'perf report'.

try:

 perf record -g -p `pidof xxx` -f
 perf report

    68.26%    :1181           b74870f2  [.] 0x000000b74870f2
              |
              |--32.09%-- 0xb73b5b44
              |          0xb7487102
              |          0xb748a4e2
              |          0xb748633d
              |          0xb73b41cd
              |          0xb73b4467
              |          0xb747d531

The reason is: for existing process, in __cmd_record(),
the pid is 0 rather than the existing process id.

Signed-off-by: Austin Zhang <austin_zhang@linux.intel.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4710.10.255.24.35.1265389362.squirrel@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>