media: mtk-jpeg: fix use-after-free in release path due to uncancelled work

The mtk_jpeg_release() function frees the context structure (ctx) without
first cancelling any pending or running work in ctx->jpeg_work. This
creates a race window where the workqueue callback may still be accessing
the context memory after it has been freed.

Race condition:

CPU 0 (release) CPU 1 (workqueue)
---------------- ------------------
close()
mtk_jpeg_release()
mtk_jpegenc_worker()
ctx = work->data
// accessing ctx

kfree(ctx) // freed!
access ctx // UAF!

The work is queued via queue_work() during JPEG encode/decode operations
(via mtk_jpeg_device_run). If the device is closed while work is pending
or running, the work handler will access freed memory.

Fix this by calling cancel_work_sync() BEFORE acquiring the mutex. This
ordering is critical: if cancel_work_sync() is called after mutex_lock(),
and the work handler also tries to acquire the same mutex, it would cause
a deadlock.

Note: The open error path does NOT need cancel_work_sync() because
INIT_WORK() only initializes the work structure - it does not schedule
it. Work is only scheduled later during ioctl operations.

Fixes: 5fb1c2361e56 ("mtk-jpegenc: add jpeg encode worker interface")
Cc: stable@vger.kernel.org
Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>