perf test: Handle properly readdir DT_UNKNOWN

Some system can return DT_UNKNOWN in readdir's struct dirent::d_type and
we must handle it properly. In this case we can directly check if the
entity we found is directory and skip it.

Reported-by: Michael Petlan <mpetlan@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171206174535.25380-1-jolsa@kernel.org


[ Split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>