debugfs: fix wait/cancellation handling during remove

In the Linux kernel, the following vulnerability has been resolved: debugfs: fix wait/cancellation handling during remove Ben Greear further reports deadlocks during concurrent debugfs remove while files are being accessed, even though the code in question now uses debugfs cancellations. Turns out that despite all the review on the locking, we missed completely that the logic is wrong: if the refcount hits zero we can finish (and need not wait for the completion), but if it doesn't we have to trigger all the cancellations. As written, we can _never_ get into the loop triggering the cancellations. Fix this, and explain it better while at it.