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/infiniband/hw/nes/nes_nic.c b/drivers/infiniband/hw/nes/nes_nic.c
index 7cb7f29..47b2ee4 100644
--- a/drivers/infiniband/hw/nes/nes_nic.c
+++ b/drivers/infiniband/hw/nes/nes_nic.c
@@ -444,10 +444,10 @@
 			skb_frag_t *frag =
 				&skb_shinfo(skb)->frags[skb_fragment_index];
 			bus_address = skb_frag_dma_map(&nesdev->pcidev->dev,
-						       frag, 0, frag->size,
+						       frag, 0, skb_frag_size(frag),
 						       DMA_TO_DEVICE);
 			wqe_fragment_length[wqe_fragment_index] =
-					cpu_to_le16(skb_shinfo(skb)->frags[skb_fragment_index].size);
+					cpu_to_le16(skb_frag_size(&skb_shinfo(skb)->frags[skb_fragment_index]));
 			set_wqe_64bit_value(nic_sqe->wqe_words, NES_NIC_SQ_WQE_FRAG0_LOW_IDX+(2*wqe_fragment_index),
 				bus_address);
 			wqe_fragment_index++;
@@ -565,7 +565,7 @@
 					&skb_shinfo(skb)->frags[tso_frag_count];
 				tso_bus_address[tso_frag_count] =
 					skb_frag_dma_map(&nesdev->pcidev->dev,
-							 frag, 0, frag->size,
+							 frag, 0, skb_frag_size(frag),
 							 DMA_TO_DEVICE);
 			}
 
@@ -637,11 +637,11 @@
 				}
 				while (wqe_fragment_index < 5) {
 					wqe_fragment_length[wqe_fragment_index] =
-							cpu_to_le16(skb_shinfo(skb)->frags[tso_frag_index].size);
+							cpu_to_le16(skb_frag_size(&skb_shinfo(skb)->frags[tso_frag_index]));
 					set_wqe_64bit_value(nic_sqe->wqe_words, NES_NIC_SQ_WQE_FRAG0_LOW_IDX+(2*wqe_fragment_index),
 						(u64)tso_bus_address[tso_frag_index]);
 					wqe_fragment_index++;
-					tso_wqe_length += skb_shinfo(skb)->frags[tso_frag_index++].size;
+					tso_wqe_length += skb_frag_size(&skb_shinfo(skb)->frags[tso_frag_index++]);
 					if (wqe_fragment_index < 5)
 						wqe_fragment_length[wqe_fragment_index] = 0;
 					if (tso_frag_index == tso_frag_count)