io-wq: cancel request if it's asking for files and we don't have them

This can't currently happen, but will be possible shortly. Handle missing
files just like we do not being able to grab a needed mm, and mark the
request as needing cancelation.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -482,6 +482,10 @@ static void io_impersonate_work(struct io_worker *worker,
 		current->files = work->identity->files;
 		current->nsproxy = work->identity->nsproxy;
 		task_unlock(current);
+		if (!work->identity->files) {
+			/* failed grabbing files, ensure work gets cancelled */
+			work->flags |= IO_WQ_WORK_CANCEL;
+		}
 	}
 	if ((work->flags & IO_WQ_WORK_FS) && current->fs != work->identity->fs)
 		current->fs = work->identity->fs;