netgraph7: Factor out and inline item reference counting code.

netgraph7: Factor out and inline item reference counting code.

* Netgraph7 assumes that nodes synchronously consume the items passed to them,
  i.e. either 1) immediatly drop the item or 2) immediatly pass the item to the
  next node.

  The previous assumption is not true for nodes that have their own internal
  item queues and defer the processing of the item.  Such nodes can use these
  routines to prevent the items from being freed too early.

* Move the apply callback check into the item reference release code.