perf thread: Rename thread->mg to thread->maps

One more step on 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-69vcr8pubpym90skxhmbwhiw@git.kernel.org


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