Count total references to a given file, disregarding "kusecount" (event listener, as with O_EVTONLY) references.
- macOS 10.5+
The vnode whose references to count.
Count of references.
For a regular file, just return (usecount-kusecount); for device files, return the sum over all vnodes 'v' which reference that device of (usecount(v) - kusecount(v)). Note that this is merely a snapshot and could be invalid by the time the caller checks the result.