perf tests: Fix typo in sample-parsing.c

It was testing the same buffer for differences:

   memcmp(s1->user_stack.data, s1->user_stack.data, s1->user_stack.size)

I'm pretty sure this wasn't supposed to be dead code.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1421946083-29863-1-git-send-email-linux@rasmusvillemoes.dkSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/sample-parsing.c

@@ -110,7 +110,7 @@ static bool samples_same(const struct perf_sample *s1,
 
 	if (type & PERF_SAMPLE_STACK_USER) {
 		COMP(user_stack.size);
-		if (memcmp(s1->user_stack.data, s1->user_stack.data,
+		if (memcmp(s1->user_stack.data, s2->user_stack.data,
 			   s1->user_stack.size)) {
 			pr_debug("Samples differ at 'user_stack'\n");
 			return false;