flusher: Fix PF_FROZEN race
To touch task->flags directly is racy. thaw_process() still has race
(changing non_current->flags, but this is another issue) though, I think
it's much better off.
So, use thaw_process() instead.
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>