CVE-2026-31583

In the Linux kernel, the following vulnerability has been resolved:

media: em28xx: fix use-after-free in em28xx_v4l2_open()

em28xx_v4l2_open() reads dev->v4l2 without holding dev->lock,
creating a race with em28xx_v4l2_init()'s error path and
em28xx_v4l2_fini(), both of which free the em28xx_v4l2 struct
and set dev->v4l2 to NULL under dev->lock.

This race leads to two issues:
 - use-after-free in v4l2_fh_init() when accessing vdev->ctrl_handler,
   since the video_device is embedded in the freed em28xx_v4l2 struct.
 - NULL pointer dereference in em28xx_resolution_set() when accessing
   v4l2->norm, since dev->v4l2 has been set to NULL.

Fix this by moving the mutex_lock() before the dev->v4l2 read and
adding a NULL check for dev->v4l2 under the lock.