field of a vnode.
the vnode to increment
Each vnode maintains a reference count of how many parts of the system
are using the vnode.
This allows the system to detect when a vnode is
no longer being used and can be safely recycled for a different file.
Any code in the system which is using a vnode (e.g. during the
operation of some algorithm or to store in a data structure) should
locks the vnode interlock while
expects the interlock to already be held.