perf map: Use zalloc for map_groups

In the next commit we will add new fields to map_groups and we need
these to be null if no value is assigned.  The simplest way to achieve
this is to request zeroed memory from the allocator.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: john keeping <john@metanate.com>
Link: http://lkml.kernel.org/r/20190815100146.28842-1-john@metanate.com


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