perf tools: Fix find tids routine by excluding "." and ".."

Introduce a filter function to skip "." and ".." directories when calculating
tid number, otherwise tid 0 will be included in the all_tid result array.

Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <4C185F68.1020505@cn.fujitsu.com>
Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>