perf/urgent fixes:

. Reference count maps in callchains, fixing a SEGFAULT when referencing a
  map after it is freed (Krister Johansen)

- Fix segfault on 'perf diff -o N' option (Namhyung Kim)

- Fix 'perf diff -o/--order' option behavior (Namhyung Kim)

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