Unlock the object cache before looking for collapse candidates.
Use vm_object_lock_try() to obtain a lock on a collapse candidate.
This avoids a deadlock due to recursively entering
vm_object_deallocate() in case a backing object goes away.
vm_object_collapse() and helper functions:
The object passed in is locked; make sure it is locked when
we leave vm_object_collapse().
XXX: Currently, the object is not unlocked during while we possibly
wait for pager data. We need to consider the effects of
simultaneous calls to vm_object_collapse() on the same object
in case unlocking is necessary.
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