[SK_BUFF]: Introduce skb_copy_from_linear_data{_offset}

To clearly state the intent of copying from linear sk_buffs, _offset being a
overly long variant but interesting for the sake of saving some bytes.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/drivers/net/dgrs.c b/drivers/net/dgrs.c
index d223c38..df62c02 100644
--- a/drivers/net/dgrs.c
+++ b/drivers/net/dgrs.c
@@ -741,7 +741,7 @@
 		}
 
 		amt = min_t(unsigned int, len, rbdp->size - count);
-		memcpy( (char *) S2H(rbdp->buf) + count, skb->data + i, amt);
+		skb_copy_from_linear_data_offset(skb, i, S2H(rbdp->buf) + count, amt);
 		i += amt;
 		count += amt;
 		len -= amt;