perf tests: Fix typo in sample-parsing.c

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.dk
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>