latencytop: fix memory leak on latency proc file

At lstats_open(), calling get_proc_task() gets task struct, but it never put.
put_task_struct() should be called when releasing.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>