perf map: Combine maps__fixup_overlappings with its only use

In the process we can kill some of the struct map->groups usage, trying
to get rid of this per-full struct map fields getting in the way of
sharing a map across father/parent processes.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-e50eqtqw3za24vmbjnqmmcs6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>