perf top: Use __map__is_kernel()

Shorter form to figure out if a given map is the kernel one and also
reduces the number of code accessing MAP__{FUNCTION,VARIABLE}, that
should go away at some point.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-rn8pexelsxpx92ce3elu3wiw@git.kernel.org


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