esp6: fix incorrect null pointer check on xo

The check for xo being null is incorrect, currently it is checking
for non-null, it should be checking for null.

Detected with CoverityScan, CID#1429349 ("Dereference after null check")

Fixes: 7862b405 ("esp: Add gso handlers for esp4 and esp6")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

net/ipv6/esp6_offload.c

@@ -120,7 +120,7 @@ static struct sk_buff *esp6_gso_segment(struct sk_buff *skb,
 	netdev_features_t esp_features = features;
 	struct xfrm_offload *xo = xfrm_offload(skb);
 
-	if (xo)
+	if (!xo)
 		goto out;
 
 	seq = xo->seq.low;