media: allegro: Fix use after free on error

The "channel" is added to the "dev->channels" but then if
v4l2_m2m_ctx_init() fails then we free "channel" but it's still on the
list so it could lead to a use after free.  Let's not add it to the
list until after v4l2_m2m_ctx_init() succeeds.

Fixes: cc62c747 ("media: allegro: add missed checks in allegro_open()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/staging/media/allegro-dvt/allegro-core.c

@@ -2483,8 +2483,6 @@ static int allegro_open(struct file *file)
 	INIT_LIST_HEAD(&channel->buffers_reference);
 	INIT_LIST_HEAD(&channel->buffers_intermediate);
 
-	list_add(&channel->list, &dev->channels);
-
 	channel->fh.m2m_ctx = v4l2_m2m_ctx_init(dev->m2m_dev, channel,
 						allegro_queue_init);
 
@@ -2493,6 +2491,7 @@ static int allegro_open(struct file *file)
 		goto error;
 	}
 
+	list_add(&channel->list, &dev->channels);
 	file->private_data = &channel->fh;
 	v4l2_fh_add(&channel->fh);