net: add skb frag size accessors

To ease skb->truesize sanitization, its better to be able to localize
all references to skb frags size.

Define accessors : skb_frag_size() to fetch frag size, and
skb_frag_size_{set|add|sub}() to manipulate it.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/ibm/ehea/ehea_main.c b/drivers/net/ethernet/ibm/ehea/ehea_main.c
index adb462d..0d4d4f6 100644
--- a/drivers/net/ethernet/ibm/ehea/ehea_main.c
+++ b/drivers/net/ethernet/ibm/ehea/ehea_main.c
@@ -1676,7 +1676,7 @@
 
 			/* copy sg1entry data */
 			sg1entry->l_key = lkey;
-			sg1entry->len = frag->size;
+			sg1entry->len = skb_frag_size(frag);
 			sg1entry->vaddr =
 				ehea_map_vaddr(skb_frag_address(frag));
 			swqe->descriptors++;
@@ -1689,7 +1689,7 @@
 			sgentry = &sg_list[i - sg1entry_contains_frag_data];
 
 			sgentry->l_key = lkey;
-			sgentry->len = frag->size;
+			sgentry->len = frag_size(frag);
 			sgentry->vaddr = ehea_map_vaddr(skb_frag_address(frag));
 			swqe->descriptors++;
 		}