perf test: Reduce the sample_freq for the 'object code reading' test

Using 4 kHz is not necessary and sometimes is more than what was
auto-tuned:

  # dmesg | grep max_sample_rate | tail -2
  [ 2499.144373] perf interrupt took too long (2501 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
  [ 3592.413606] perf interrupt took too long (5069 > 5000), lowering kernel.perf_event_max_sample_rate to 25000

Simulating a auto-tune of 2000 we make the test fail, as reported
by Steven Noonan for one of his machines, so reduce it to 500 HZ,
it is enough to get a good number of samples for this test:

  # perf test -v 21 2>&1  | grep '^Reading object code for memory address' | tee /tmp/out | tail -5
  Reading object code for memory address: 0x479f40
  Reading object code for memory address: 0x7f29b7eea80d
  Reading object code for memory address: 0x7f29b7eea80d
  Reading object code for memory address: 0x7f29b7eea800
  Reading object code for memory address: 0xffffffff813b2f23
  [root@jouet ~]# wc -l /tmp/out
  40 /tmp/out
  [root@jouet ~]#

For systems that auto-tune below that, the previous patches will tell the
user what is happening so that he may either ignore the result of this test or
bump /proc/sys/kernel/perf_event_max_sample_rate.

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-6kufyy1iprdfzrbtuqgxir70@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/code-reading.c

@@ -439,7 +439,7 @@ static int do_test_code_reading(bool try_kcore)
 		.mmap_pages	     = UINT_MAX,
 		.user_freq	     = UINT_MAX,
 		.user_interval	     = ULLONG_MAX,
-		.freq		     = 4000,
+		.freq		     = 500,
 		.target		     = {
 			.uses_mmap   = true,
 		},