[media] v4l: Share code between video_usercopy and video_ioctl2
The two functions are mostly identical. They handle the copy_from_user
and copy_to_user operations related with V4L2 ioctls and call the real
ioctl handler.
Create a __video_usercopy function that implements the core of
video_usercopy and video_ioctl2, and call that function from both.