perf diff: Add missing handler for PERF_RECORD_MMAP2 events

Without mmap2, perf diff fails to find the symbol name. The default
symbol sort key doesn't work well.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1416328700-1836-2-git-send-email-kan.liang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>