Fix "shutdown system test crashed in dns_dispatchmgr_getblackhole"

Fix "shutdown system test crashed in dns_dispatchmgr_getblackhole"

While shutting down view->dispatchmgr is no longer valid.  Attach
to it and when creates a fetch context and use that pointer instead
of view->dispatchmgr.  Use dns_view_getdispatchmgr to do the attaching
as view->dispatchmgr is it managed using rcu.