perf machine: Add method for common kernel_map(FUNCTION) operation

And it is also a step in the direction of killing the separation of data
and text maps in map_groups.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-rrds86kb3wx5wk8v38v56gw8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-kmem.c

@@ -329,7 +329,7 @@ static int build_alloc_func_list(void)
 		return -EINVAL;
 	}
 
-	kernel_map = machine__kernel_map(machine, MAP__FUNCTION)
+	kernel_map = machine__kernel_map(machine);
 	if (map__load(kernel_map, NULL) < 0) {
 		pr_err("cannot load kernel map\n");
 		return -ENOENT;