Increment open file count manually

For engines/logs that manually add files, be sure to increment
the open file count.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/net.c b/engines/net.c
index 110e158..fcf4b89 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -1196,6 +1196,7 @@
 	if (!td->files_index) {
 		add_file(td, td->o.filename ?: "net", 0, 0);
 		td->o.nr_files = td->o.nr_files ?: 1;
+		td->o.open_files++;
 	}
 
 	if (!td->io_ops->data) {