function increases the hold count on a page.
This prevents the page daemon from freeing the page.
should only be used for very temporary wiring of a page,
as that page will not be considered for paging or
reallocation for as long as its hold count is greater
Also note that while wired pages are removed from whatever
queue they are on,
does not affect the location of the page.
If it is on a queue prior to the call, it will still
be there afterward.
If the page needs to be held for a long period of time,
should be used.
function reduces the hold count on a page.
If the hold count is zero it is possible that the page will be freed by the