Convert from ioctl to unlocked_ioctl using the v4l2 core lock.

Also removed the 'exclusive access' limitation. There was no need for it
and it violates the v4l2 spec as well.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>