perf unwind: Use find_map function in access_dso_mem

perf unwind: Use find_map function in access_dso_mem

The find_map helper is already there, so let's use it.

Also we're going to introduce wider search in following patch, so it'll
be easier to make this change on single place.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Noel Grandin <noelgrandin@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452158050-28061-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>