v4l2allocator: allow importing single-planar dmabufs with padded height
When exporting a dmabuf from a v4l2decoder, memory size is determined by the padded height. Configuring a v4l2 importer to the non-padded height causes a mismatch between queue sizeimage calculated with the non-padded height and bytesused set to the memory size. Since the importer will only ever read up to queue sizeimage, it should be safe to just clamp bytesused to sizeimage.