perf tests: Use perf_evlist__strerror_open() to provide hints about max_freq

Before:

  # perf test -v "code reading" 2>&1 | tail -4
  perf_evlist__open failed
  test child finished with -1
  ---- end ----
  Test object code reading: FAILED!
  #

After:

  # perf test -v "code reading" 2>&1 | tail -7
  perf_evlist__open() failed!
  Error: Invalid argument.
  Hint:  Check /proc/sys/kernel/perf_event_max_sample_rate.
  Hint:  The current value is 1000 and 4000 is being requested.
  test child finished with -1
  ---- end ----
  Test object code reading: FAILED!
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Noonan <steven@uplinklabs.net>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ifbx7vmrc38loe6317owz2jx@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/code-reading.c

@@ -559,7 +559,13 @@ static int do_test_code_reading(bool try_kcore)
 				evlist = NULL;
 				continue;
 			}
-			pr_debug("perf_evlist__open failed\n");
+
+			if (verbose) {
+				char errbuf[512];
+				perf_evlist__strerror_open(evlist, errno, errbuf, sizeof(errbuf));
+				pr_debug("perf_evlist__open() failed!\n%s\n", errbuf);
+			}
+
 			goto out_put;
 		}
 		break;