net: move zerocopy_sg_from_iovec() to net/core/datagram.c

To let it be reused and reduce code duplication. Also document this function.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ae46475..5ac96f3 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2359,6 +2359,10 @@
 						    const struct iovec *from,
 						    int from_offset,
 						    int len);
+extern int	       zerocopy_sg_from_iovec(struct sk_buff *skb,
+					      const struct iovec *frm,
+					      int offset,
+					      size_t count);
 extern int	       skb_copy_datagram_const_iovec(const struct sk_buff *from,
 						     int offset,
 						     const struct iovec *to,