Fix a bug in global_cached_io.
Fix a bug in global_cached_io.

There is a race condition if I don't use mutex here.

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