xen/netback: Pass (void *) to virt_to_page()

virt_to_page() takes a virtual address as argument but
the driver passes an unsigned long, which works because
the target platform(s) uses polymorphic macros to calculate
the page.

Since many architectures implement virt_to_pfn() as
a macro, this function becomes polymorphic and accepts both a
(unsigned long) and a (void *).

Fix this up by an explicit (void *) cast.

Cc: Wei Liu <wei.liu@kernel.org>
Cc: Paul Durrant <paul@xen.org>
Cc: xen-devel@lists.xenproject.org
Cc: netdev@vger.kernel.org
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/net/xen-netback/netback.c

@@ -689,7 +689,7 @@ static void xenvif_fill_frags(struct xenvif_queue *queue, struct sk_buff *skb)
 		prev_pending_idx = pending_idx;
 
 		txp = &queue->pending_tx_info[pending_idx].req;
-		page = virt_to_page(idx_to_kaddr(queue, pending_idx));
+		page = virt_to_page((void *)idx_to_kaddr(queue, pending_idx));
 		__skb_fill_page_desc(skb, i, page, txp->offset, txp->size);
 		skb->len += txp->size;
 		skb->data_len += txp->size;