perf tests: Set evlist of test__task_exit() to !overwrite

Changing ringbuffer to !overwrite in this task is harmless because
this test uses a very low frequency (1) and using a very simple program
(true). There should have only 3 events in the whole test.  Overwriting
is impossible to happen.
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20171113013809.212417-6-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/task-exit.c

@@ -97,7 +97,7 @@ int test__task_exit(struct test *test __maybe_unused, int subtest __maybe_unused
 		goto out_delete_evlist;
 	}
 
-	if (perf_evlist__mmap(evlist, 128, true) < 0) {
+	if (perf_evlist__mmap(evlist, 128, false) < 0) {
 		pr_debug("failed to mmap events: %d (%s)\n", errno,
 			 str_error_r(errno, sbuf, sizeof(sbuf)));
 		goto out_delete_evlist;