[media] v4l2: move tracepoints to video_usercopy

The (d)qbuf ioctls were traced in the low-level v4l2 ioctl function. The
trace was outside the serialization lock, so that can affect the usefulness
of the timing. In addition, the __user pointer was expected instead of a
proper kernel pointer.

By moving the tracepoints to video_usercopy we ensure that the trace calls
use the correct kernel pointer, and that it happens right after the ioctl
call to the driver, so certainly inside the serialization lock.

In addition, we only trace if the call was successful.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Wade Farnsworth <wade_farnsworth@mentor.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>