This patch modify discard thread wait policy as below:
	issued       io_interrupted     wait time(ms)
1.        8                 0               50
2.      (0,8)               1               50
3.        0                 1              500 (dev is busy)
4.        0                 0            60000 (no candidates)
Signed-off-by: NYunlei He <heyunlei@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>