perf maps: Add for_each_entry()/_safe() iterators

To reduce boilerplate, provide a more compact form using an idiom
present in other trees of data structures.

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-59gmq4kg1r68ou1wknyjl78x@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/x86/util/event.c

@@ -29,7 +29,7 @@ int perf_event__synthesize_extra_kmaps(struct perf_tool *tool,
 		return -1;
 	}
 
-	for (pos = maps__first(maps); pos; pos = map__next(pos)) {
+	maps__for_each_entry(maps, pos) {
 		struct kmap *kmap;
 		size_t size;