Allow wquque to use callback to free an mbuf - so

mbufs can be reused/pooled

include/ucore/wqueue.h

@@ -67,6 +67,12 @@ typedef enum UC_WQ_RESULT (*wqueue_write_cb)(struct IOMux *mux,
 typedef enum UC_WQ_RESULT (*wqueue_read_cb)(struct IOMux *mux, 
                                             struct UCWQueue *wqueue);
 
+/**
+ * Callback to free a MBuf once it's fully written.
+ * @param the fd of the current wqueue
+ */
+typedef void (*wqueue_free_mbuf_cb)(struct MBuf *mbuf);
+
 /** 
  * Represents a queue of data to write.
  * UCWQueue replaces a raw IOMuxFD, and manages a queue
@@ -91,7 +97,8 @@ struct UCWQueue {
     /** Users callback for write events
      */
     wqueue_write_cb write_cb;
-
+    /** Function used to free an MBuf - default is uc_mbuf_free()*/
+    wqueue_free_mbuf_cb free_cb;
     /** Current number of queued struct MBuf
      */
     unsigned int queue_len;