Use the V4L2 core locking system instead of rolling your own. Switch to
the vb2 fop and ioctl helpers to get rid of a lot of code. This also made
it easy to add VB2_READ to the DMA modes, since you get read() for free
with vb2 and these helpers.

Finally remove the users field: this information is also available from
the core framework, no need to keep track of it in the driver.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>