Push down the page queues lock inside of vm_page_free_toq() and

Push down the page queues lock inside of vm_page_free_toq() and
pmap_page_is_mapped() in preparation for removing page queues locking
around calls to vm_page_free().  Setting aside the assertion that calls
pmap_page_is_mapped(), vm_page_free_toq() now acquires and holds the page
queues lock just long enough to actually add or remove the page from the
paging queues.

Update vm_page_unhold() to reflect the above change.