We add the number of stripes on target devices into bbio->num_stripes
if we are under device replacement, and we just sort the raid_map of
those stripes that not on the target devices, so if when we need
real raid_map, we need skip the stripes on the target devices.
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com> Signed-off-by: Chris Mason <clm@fb.com>