perf map_groups: Introduce for_each_entry() and for_each_entry_safe() iterators

To reduce boilerplate, providing a more compact form to iterate over the
maps in a map_group.

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-gc3go6fmdn30twusg91t2q56@git.kernel.org


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