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/net/core/dev.c b/net/core/dev.c
index 8b6118a..cbb5918 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3489,9 +3489,9 @@
 		skb->data_len -= grow;
 
 		skb_shinfo(skb)->frags[0].page_offset += grow;
-		skb_shinfo(skb)->frags[0].size -= grow;
+		skb_frag_size_sub(&skb_shinfo(skb)->frags[0], grow);
 
-		if (unlikely(!skb_shinfo(skb)->frags[0].size)) {
+		if (unlikely(!skb_frag_size(&skb_shinfo(skb)->frags[0]))) {
 			skb_frag_unref(skb, 0);
 			memmove(skb_shinfo(skb)->frags,
 				skb_shinfo(skb)->frags + 1,
@@ -3559,7 +3559,7 @@
 	    !PageHighMem(skb_frag_page(&skb_shinfo(skb)->frags[0]))) {
 		NAPI_GRO_CB(skb)->frag0 =
 			skb_frag_address(&skb_shinfo(skb)->frags[0]);
-		NAPI_GRO_CB(skb)->frag0_len = skb_shinfo(skb)->frags[0].size;
+		NAPI_GRO_CB(skb)->frag0_len = skb_frag_size(&skb_shinfo(skb)->frags[0]);
 	}
 }
 EXPORT_SYMBOL(skb_gro_reset_offset);