datagram: open-code copy_page_to_iter

This will be useful to consolidate skb_copy_and_hash_datagram_iter and
skb_copy_and_csum_datagram to a single code path.
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sagi Grimberg <sagi@lightbitslabs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

net/core/datagram.c

@@ -445,11 +445,14 @@ int skb_copy_datagram_iter(const struct sk_buff *skb, int offset,
 
 		end = start + skb_frag_size(frag);
 		if ((copy = end - offset) > 0) {
+			struct page *page = skb_frag_page(frag);
+			u8 *vaddr = kmap(page);
+
 			if (copy > len)
 				copy = len;
-			n = copy_page_to_iter(skb_frag_page(frag),
-					      frag->page_offset + offset -
-					      start, copy, to);
+			n = copy_to_iter(vaddr + frag->page_offset +
+					 offset - start, copy, to);
+			kunmap(page);
 			offset += n;
 			if (n != copy)
 				goto short_copy;