perf map: Add map__refcnt() accessor to use in the maps test

perf map: Add map__refcnt() accessor to use in the maps test

To remove one more direct access to 'struct map' so that we can intecept
accesses to its instantiations and refcount check it to catch use after
free, etc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Riccardo Mancini <rickyman7@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Link: https://lore.kernel.org/lkml/ZDbRIJknafLnDwtO@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>