virtio_random: avoid using vtrnd_detach in the failure path

On the off chance that we discover more than one virtio-random device,
the second one will deregister the global instance or trip an invariant
when it fails to attach and calls vtrnd_detach().

We don't have that many resources to free, so just inline those.  This
is purely a hygiene thing, we haven't seen an instance of this in the
wild (to my knowledge).

Reviewed by:	bnovkov, kfv
Sponsored by:	Stormshield
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D61050

sys/dev/virtio/random/virtio_random.c

@@ -184,8 +184,15 @@ vtrnd_attach(device_t dev)
 	vtrnd_enqueue(sc);
 
 fail:
-	if (error)
-		vtrnd_detach(dev);
+	if (error != 0) {
+		/* Parent will free the virtqueue. */
+		if (g_vtrnd_softc == sc) {
+			atomic_store_explicit(&g_vtrnd_softc, NULL,
+			    memory_order_release);
+		}
+		sglist_free(sc->vtrnd_sg);
+		zfree(sc->vtrnd_value, M_DEVBUF);
+	}
 
 	return (error);
 }