media: v4l2-core: split out data copy from video_usercopy

The copy-in/out portions of video_usercopy() are about to
get more complex, so turn then into separate functions as
a cleanup first.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>