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

If the queue of requests generated by user tasks is full, don't need to
do anything on the incomplete user tasks.

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