xen-netback: use local var in xenvif_tx_check_gop() instead of re-calculating

shinfo already holds the result of skb_shinfo(skb) at this point - no
need to re-invoke the construct even twice.
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -557,8 +557,8 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 	}
 
 	if (skb_has_frag_list(skb) && !first_shinfo) {
-		first_shinfo = skb_shinfo(skb);
-		shinfo = skb_shinfo(skb_shinfo(skb)->frag_list);
+		first_shinfo = shinfo;
+		shinfo = skb_shinfo(shinfo->frag_list);
 		nr_frags = shinfo->nr_frags;
 
 		goto check_frags;