perf stat: Measure 't0' and 'ref_time' after enable_counters()

Take measurements of 't0' and 'ref_time' after enable_counters(), so
that they only measure the time consumed when the counters are enabled.

Signed-off-by: Song Liu <[e-mail]>
Acked-by: Andi Kleen <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-stat.c

···
 	/*
 	 * Enable counters and exec the command:
 	 */
-	t0 = rdclock();
-	clock_gettime(CLOCK_MONOTONIC, &ref_time);
-
 	if (forks) {
 		evlist__start_workload(evsel_list);
 		err = enable_counters();
 		if (err)
 			return -1;
+
+		t0 = rdclock();
+		clock_gettime(CLOCK_MONOTONIC, &ref_time);
 
 		if (interval || timeout || evlist__ctlfd_initialized(evsel_list))
 			status = dispatch_events(forks, timeout, interval, &times);
···
 		err = enable_counters();
 		if (err)
 			return -1;
+
+		t0 = rdclock();
+		clock_gettime(CLOCK_MONOTONIC, &ref_time);
+
 		status = dispatch_events(forks, timeout, interval, &times);
 	}