net/mlx4_en: do batched put_page using atomic_sub

This patch fixes couple error paths after allocation failures.
Atomic set of page reference counter is safe only if it is zero,
otherwise set can race with any speculative get_page_unless_zero.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -126,7 +126,9 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
 			dma_unmap_page(priv->ddev, page_alloc[i].dma,
 				page_alloc[i].page_size, PCI_DMA_FROMDEVICE);
 			page = page_alloc[i].page;
-			set_page_count(page, 1);
+			/* Revert changes done by mlx4_alloc_pages */
+			page_ref_sub(page, page_alloc[i].page_size /
+					   priv->frag_info[i].frag_stride - 1);
 			put_page(page);
 		}
 	}
@@ -176,7 +178,9 @@ static int mlx4_en_init_allocator(struct mlx4_en_priv *priv,
 		dma_unmap_page(priv->ddev, page_alloc->dma,
 			       page_alloc->page_size, PCI_DMA_FROMDEVICE);
 		page = page_alloc->page;
-		set_page_count(page, 1);
+		/* Revert changes done by mlx4_alloc_pages */
+		page_ref_sub(page, page_alloc->page_size /
+				   priv->frag_info[i].frag_stride - 1);
 		put_page(page);
 		page_alloc->page = NULL;
 	}