static struct mdev_driver vfio_mdev_driver = {
+	.driver = {
+		.name = "vfio_mdev",
+		.owner = THIS_MODULE,
+		.mod_name = KBUILD_MODNAME,
+	},

What is the mod_name initialization for?  I've not really seen
that in anywere else, and the only user seems to be
module_add_driver for a rather odd case we shouldn't hit here.

The rest looks good to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>