Sprinkle uvm_obj_destroy() over UVM object recycling code.

Sprinkle uvm_obj_destroy() over UVM object recycling code.

For now, only assert that the tree of pages is empty in uvm_obj_destroy().
This will soon be used to free the per-UVM object lock.

While here call uvm_obj_init() when new vnodes are allocated instead of
in uvn_attach().  Because vnodes and there associated UVM object are
currently never freed, it isn't easy to know where/when to garbage
collect the associated lock.  So simply check that the reference of a
given object is 0 when uvn_attach().

Tested by many as part of a bigger diff.

ok kettenis@