perf machine: Use machine__kernel_map() thoroughly

In places where we were using its open coded equivalent.

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-khkdugcdoqy3tkszm3jdxgbe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/code-reading.c

@@ -473,7 +473,7 @@ static int do_test_code_reading(bool try_kcore)
 		symbol_conf.kallsyms_name = "/proc/kallsyms";
 
 	/* Load kernel map */
-	map = machine->vmlinux_maps[MAP__FUNCTION];
+	map = machine__kernel_map(machine, MAP__FUNCTION);
 	ret = map__load(map, NULL);
 	if (ret < 0) {
 		pr_debug("map__load failed\n");