net: page pool: allow to pass zero flags to page_pool_init()

page pool API can be useful for non-DMA cases like
xen-netfront driver so let's allow to pass zero flags to
page pool flags.

v2: check DMA direction only if PP_FLAG_DMA_MAP is set
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/page_pool.c

@@ -43,9 +43,11 @@ static int page_pool_init(struct page_pool *pool,
 	 * DMA_BIDIRECTIONAL is for allowing page used for DMA sending,
 	 * which is the XDP_TX use-case.
 	 */
+	if (pool->p.flags & PP_FLAG_DMA_MAP) {
 		if ((pool->p.dma_dir != DMA_FROM_DEVICE) &&
 		    (pool->p.dma_dir != DMA_BIDIRECTIONAL))
 			return -EINVAL;
+	}
 
 	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV) {
 		/* In order to request DMA-sync-for-device the page