t/io_uring: only calculate per-file depth if we have files

If NOPs are used, then no files exist, and hence the app will throw
a floating point error when trying to divide by zero.

Fixes: 6067863 ("t/io_uring: pre-calculate per-file depth")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

t/io_uring.c

@@ -974,7 +974,9 @@ static int setup_ring(struct submitter *s)
 	for (i = 0; i < p.sq_entries; i++)
 		sring->array[i] = i;
 
-	s->per_file_depth = (depth + s->nr_files - 1) / s->nr_files;
+	s->per_file_depth = INT_MAX;
+	if (s->nr_files)
+		s->per_file_depth = (depth + s->nr_files - 1) / s->nr_files;
 	return 0;
 }