net: skb_copy_datagram_from_iovec()

There's an skb_copy_datagram_iovec() to copy out of a paged skb, but
nothing the other way around (because we don't do that).

We want to allocate big skbs in tun.c, so let's add the function.
It's a carbon copy of skb_copy_datagram_iovec() with enough changes to
be annoying.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 358661c..9099237 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1452,6 +1452,10 @@
 extern int	       skb_copy_and_csum_datagram_iovec(struct sk_buff *skb,
 							int hlen,
 							struct iovec *iov);
+extern int	       skb_copy_datagram_from_iovec(struct sk_buff *skb,
+						    int offset,
+						    struct iovec *from,
+						    int len);
 extern void	       skb_free_datagram(struct sock *sk, struct sk_buff *skb);
 extern int	       skb_kill_datagram(struct sock *sk, struct sk_buff *skb,
 					 unsigned int flags);