perf maps: Rename 'mg' variables to 'maps'

Continuing the merge of 'struct maps' with 'struct map_groups'.

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-z8d14wrw393a0fbvmnk1bqd9@git.kernel.org


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