When the VM needs to allocated physical memory pages (for non interrupt use)

When the VM needs to allocated physical memory pages (for non interrupt use)
and it has not plenty of free pages it tries to free pages in the cache queue.
Unfortunately freeing a cached page requires the locking of the object that
owns the page. However in the context of allocating pages we may not be able
to lock the object and thus can only TRY to lock the object. If the locking try
fails the cache page can not be freed and is activated to move it out of the way
so that we may try to free other cache pages.

If all pages in the cache belong to objects that are currently locked the
cache queue can be emptied without freeing a single page. This scenario caused
two problems:

    1)  vm_page_alloc always failed allocation when it tried freeing pages from
        the cache queue and failed to do so. However if there are more than
        cnt.v_interrupt_free_min pages on the free list it should return pages
        when requested with priority VM_ALLOC_SYSTEM. Failure to do so can cause
        resource exhaustion deadlocks.

    2)  Threads than need to allocate pages spend a lot of time cleaning up the
        page queue without really getting anything done while the pagedaemon
         needs to work overtime to refill the cache.

This change fixes the first problem. (1)

Reviewed by:	tegge@