media: em28xx: Fix race condition between open and init function

Fixes a race condition - for lack of a more precise term - between
em28xx_v4l2_open and em28xx_v4l2_init, by detaching the v4l2_dev,
media_pad and vdev structs from the em28xx_v4l2, and managing the
lifetime of those objects more dynamicaly.

The race happens when a thread[1] - containing the em28xx_v4l2_init()
code - calls the v4l2_mc_create_media_graph(), and it return a error,
if a thread[2] - running v4l2_open() - pass the verification point
and reaches the em28xx_v4l2_open() before the thread[1] finishes
the deregistration of v4l2 subsystem, the thread[1] will free all
resources before the em28xx_v4l2_open() can process their things,
because the em28xx_v4l2_init() has the dev->lock. And all this lead
the thread[2] to cause a user-after-free.

Reported-and-tested-by: syzbot+b2391895514ed9ef4a8e@syzkaller.appspotmail.com
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>

drivers/media/usb/em28xx/em28xx-camera.c

@@ -340,7 +340,7 @@ int em28xx_init_camera(struct em28xx *dev)
 		v4l2->sensor_xtal = 4300000;
 		pdata.xtal = v4l2->sensor_xtal;
 		if (NULL ==
-		    v4l2_i2c_new_subdev_board(&v4l2->v4l2_dev, adap,
+		    v4l2_i2c_new_subdev_board(v4l2->v4l2_dev, adap,
 					      &mt9v011_info, NULL))
 			return -ENODEV;
 		v4l2->vinmode = EM28XX_VINMODE_RGB8_GRBG;
@@ -394,7 +394,7 @@ int em28xx_init_camera(struct em28xx *dev)
 		v4l2->sensor_yres = 480;
 
 		subdev =
-		     v4l2_i2c_new_subdev_board(&v4l2->v4l2_dev, adap,
+		     v4l2_i2c_new_subdev_board(v4l2->v4l2_dev, adap,
 					       &ov2640_info, NULL);
 		if (!subdev)
 			return -ENODEV;