Old code used bbio->raid_map to determine whether in raid56
write/recover operation, because we didn't't have bbio->map_type.

Now we have direct way for this condition, rid of using
the function-relative data, and make the code more readable.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>