perf test: Report failure for mmap events

When fail to mmap events in task exit case, it misses to set 'err' to
-1; thus the testing will not report failure for it.

This patch sets 'err' to -1 when fails to mmap events, thus Perf tool
can report correct result.

Fixes: d723a55096 ("perf test: Add test case for checking number of EXIT events")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20191011091942.29841-1-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/task-exit.c

@@ -111,6 +111,7 @@ int test__task_exit(struct test *test __maybe_unused, int subtest __maybe_unused
 	if (evlist__mmap(evlist, 128) < 0) {
 		pr_debug("failed to mmap events: %d (%s)\n", errno,
 			 str_error_r(errno, sbuf, sizeof(sbuf)));
+		err = -1;
 		goto out_delete_evlist;
 	}