[SAFS]: fix a bug global_cached_io.
[SAFS]: fix a bug global_cached_io.

previously, global_cached_io can't issue a large read request to the
underlying IO due to the wrong RAID block size.

icoming/FlashX,silky/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashX,silky/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,silky/FlashGraph,icoming/FlashX,silky/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,silky/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,flashxio/FlashX