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 <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jean Pihet <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Jiri Olsa <[email protected]>