perf evlist: Fix creation of cpu map

Currently, 'perf record -- sleep 1' creates a cpu map for all online
cpus since it turns out calling cpu_map__new(NULL). Fix it.

Also it is guaranteed that cpu_list is NULL if PID/TID is given by
calling perf_target__validate(), so we can make the conditional bit
simpler.

This also fixes perf test 7 (Validate) failure on my 6 core machine:

  $ cat /sys/devices/system/cpu/online
  0-11
  $ ./perf test -v 7
   7: Validate PERF_RECORD_* events & perf_sample fields:
  --- start ---
  perf_evlist__mmap: Operation not permitted
  ---- end ----
  Validate PERF_RECORD_* events & perf_sample fields: FAILED!

Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1336367344-28071-3-git-send-email-namhyung.kim@lge.com


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