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>

drivers/scsi/fcoe/fcoe_transport.c

@@ -105,7 +105,7 @@ u32 fcoe_fc_crc(struct fc_frame *fp)
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 		frag = &skb_shinfo(skb)->frags[i];
 		off = frag->page_offset;
-		len = frag->size;
+		len = skb_frag_size(frag);
 		while (len > 0) {
 			clen = min(len, PAGE_SIZE - (off & ~PAGE_MASK));
 			data = kmap_atomic(