It can avoid complex calculation of real stripes in sort,
moreover, we can clean up code of sorting tgtdev_map because it
will be in order initially.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>