media: vb2: add vb2_find_timestamp()

Use v4l2_timeval_to_ns instead of timeval_to_ns to ensure that
both kernelspace and userspace will use the same conversion
function.

Next add a new vb2_find_timestamp() function to find buffers
with a specific timestamp.

This function will only look at DEQUEUED and DONE buffers, i.e.
buffers that are already processed.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>