net: page_pool: add helper function for retrieving dma direction

Since the dma direction is stored in page pool params, offer an API
helper for driver that choose not to keep track of it locally

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>