After a rsrc/files reference node's refs are killed, it must never be
used. And that's how it works, it either assigns a new node or kills the
whole data table.

Let's explicitly NULL it, that shouldn't be necessary, but if something
would go wrong I'd rather catch a NULL dereference to using a dangling
pointer.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>