camera: Implement user space pmem buffer using V4L2 userptr.

In order to support user space pmem buffer across processes,
we use V4L2 userptr buffer, in which we pass file descriptor
and offset to the driver.

Change-Id: Icf89af9b39d2223d9f6ef4beb4c2fb7c67219adc
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>
[davidb: only drivers/media/video/videobuf-core.c]
Signed-off-by: David Brown <davidb@codeaurora.org>