media: dvb_frontend: only use kref after initialized

As reported by Laurent, when a DVB frontend need to register
two drivers (e. g. a tuner and a demod), if the second driver
fails to register (for example because it was not compiled),
the error handling logic frees the frontend by calling
dvb_frontend_detach(). That used to work fine, but changeset
1f862a68df24 ("[media] dvb_frontend: move kref to struct dvb_frontend")
added a kref at struct dvb_frontend. So, now, instead of just
freeing the data, the error handling do a kref_put().

That works fine only after dvb_register_frontend() succeeds.

While it would be possible to add a helper function that
would be initializing earlier the kref, that would require
changing every single DVB frontend on non-trivial ways, and
would make frontends different than other drivers.

So, instead of doing that, let's focus on the real issue:
only call kref_put() after kref_init(). That's easy to
check, as, when the dvb frontend is successfuly registered,
it will allocate its own private struct. So, if such
struct is allocated, it means that it is safe to use
kref_put(). If not, then nobody is using yet the frontend,
and it is safe to just deallocate it.

Fixes: 1f862a68df24 ("[media] dvb_frontend: move kref to struct dvb_frontend")

Reported-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-core/dvb_frontend.c

@@ -141,22 +141,39 @@ struct dvb_frontend_private {
 static void dvb_frontend_invoke_release(struct dvb_frontend *fe,
 					void (*release)(struct dvb_frontend *fe));
 
-static void dvb_frontend_free(struct kref *ref)
+static void __dvb_frontend_free(struct dvb_frontend *fe)
 {
-	struct dvb_frontend *fe =
-		container_of(ref, struct dvb_frontend, refcount);
 	struct dvb_frontend_private *fepriv = fe->frontend_priv;
 
+	if (!fepriv)
+		return;
+
 	dvb_free_device(fepriv->dvbdev);
 
 	dvb_frontend_invoke_release(fe, fe->ops.release);
 
 	kfree(fepriv);
+	fe->frontend_priv = NULL;
+}
+
+static void dvb_frontend_free(struct kref *ref)
+{
+	struct dvb_frontend *fe =
+		container_of(ref, struct dvb_frontend, refcount);
+
+	__dvb_frontend_free(fe);
 }
 
 static void dvb_frontend_put(struct dvb_frontend *fe)
 {
-	kref_put(&fe->refcount, dvb_frontend_free);
+	/*
+	 * Check if the frontend was registered, as otherwise
+	 * kref was not initialized yet.
+	 */
+	if (fe->frontend_priv)
+		kref_put(&fe->refcount, dvb_frontend_free);
+	else
+		__dvb_frontend_free(fe);
 }
 
 static void dvb_frontend_get(struct dvb_frontend *fe)