perf top: Filter symbols based on __map__is_kernel(map)

Instead of using dso->kernel, this is equivalent at the moment,
and helps in reducing the accesses to dso->kernel.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-1pc2v63iphtifovw3bv0bo1v@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -655,7 +655,7 @@ static int symbol_filter(struct map *map, struct symbol *sym)
 {
 	const char *name = sym->name;
 
-	if (!map->dso->kernel)
+	if (!__map__is_kernel(map))
 		return 0;
 	/*
 	 * ppc64 uses function descriptors and appends a '.' to the