perf tests x86: Fix stack map lookup in dwarf unwind test

perf tests x86: Fix stack map lookup in dwarf unwind test

Previous commit 'perf x86: Fix perf to use non-executable stack, again'
moved stack map into MAP__VARIABLE map type again. Fixing the dwarf
unwind test stack map lookup appropriately.

Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Link: http://lkml.kernel.org/n/tip-ttzyhbe4zls24z7ednkmhvxl@git.kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>