perf stat: Fix endless wait for child process

We hit a 'perf stat' issue by using following script:

  #!/bin/bash

  sleep 1000 &
  exec perf stat -a -e cycles -I1000 -- sleep 5

Since "perf stat" is launched by exec, the "sleep 1000" would be the
child process of "perf stat". The wait4() call will not return because
it's waiting for the child process "sleep 1000" to end. So 'perf stat'
doesn't return even after 5s passes.

This patch lets 'perf stat' return when the specified child process ends
(in this case, the specified child process is "sleep 5").

Committer testing:

  # cat test.sh
  #!/bin/bash

  sleep 10 &
  exec perf stat -a -e cycles -I1000 -- sleep 5
  #

Before:

  # time ./test.sh
  #           time             counts unit events
       1.001113090        108,453,351      cycles
       2.002062196        142,075,435      cycles
       3.002896194        164,801,068      cycles
       4.003731666        107,062,140      cycles
       5.002068867        112,241,832      cycles

  real	0m10.066s
  user	0m0.016s
  sys	0m0.101s
  #

After:

  # time ./test.sh
  #           time             counts unit events
       1.001016096         91,412,027      cycles
       2.002014963        124,063,708      cycles
       3.002883964        125,993,929      cycles
       4.003706470        120,465,734      cycles
       5.002006778        163,560,355      cycles

  real	0m5.123s
  user	0m0.014s
  sys	0m0.105s
  #

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1546501245-4512-1-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -561,7 +561,8 @@ try_again:
 					break;
 			}
 		}
-		wait4(child_pid, &status, 0, &stat_config.ru_data);
+		if (child_pid != -1)
+			wait4(child_pid, &status, 0, &stat_config.ru_data);
 
 		if (workload_exec_errno) {
 			const char *emsg = str_error_r(workload_exec_errno, msg, sizeof(msg));