perf test: Use cpu_map->[cpu] when setting affinity

When some of CPUs are offline:

 # cat /sys/devices/system/cpu/online
 0,6-31

perf test will fail on #3 testcase:

   3: detect open syscall event on all cpus:
   --- start ---
   perf_evsel__read_on_cpu: expected to intercept 111 calls on cpu 0, got 681
   perf_evsel__read_on_cpu: expected to intercept 112 calls on cpu 1, got 117
   perf_evsel__read_on_cpu: expected to intercept 113 calls on cpu 2, got 118
   perf_evsel__read_on_cpu: expected to intercept 114 calls on cpu 3, got 119
   perf_evsel__read_on_cpu: expected to intercept 115 calls on cpu 4, got 120
   perf_evsel__read_on_cpu: expected to intercept 116 calls on cpu 5, got 121
   perf_evsel__read_on_cpu: expected to intercept 117 calls on cpu 6, got 122
   perf_evsel__read_on_cpu: expected to intercept 118 calls on cpu 7, got 123
   perf_evsel__read_on_cpu: expected to intercept 119 calls on cpu 8, got 124
   perf_evsel__read_on_cpu: expected to intercept 120 calls on cpu 9, got 125
   perf_evsel__read_on_cpu: expected to intercept 121 calls on cpu 10, got 126
   ....

This patch try to use 'cpus->map[cpu]' when setting cpu affinity, and
will check the return code of sched_setaffinity()

LKML-Reference: <20110120114707.GA11781@hpt.nay.redhat.com>
Signed-off-by: Han Pingtian <phan@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>