media: v4l2-core: Avoid the dangling pointer in v4l2_fh_release

[ Upstream commit 7dd0c9e5 ]

A use after free bug caused by the dangling pointer
filp->privitate_data in v4l2_fh_release.
See https://lore.kernel.org/patchwork/patch/1419058/

.

My patch sets the dangling pointer to NULL to provide
robust.

Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I9065ea5176a1fed1a78bd71d9761aed9360ad03e