[SAFS]: change num_pending_ios() in global_cached_io.
[SAFS]: change num_pending_ios() in global_cached_io.

Exclude the number of incomplete user tasks from the number of pending
I/O requests. Otherwise, it may cause deadlock in FlashGraph (user tasks
can't be completed, but they can't issue new I/O requests because they
don't know data locations they request).

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