ipv6: esp6: use BUG_ON instead of if condition followed by BUG

Use BUG_ON instead of if condition followed by BUG in esp_remove_trailer.

This issue was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/esp6.c

@@ -483,8 +483,8 @@ static inline int esp_remove_trailer(struct sk_buff *skb)
 		goto out;
 	}
 
-	if (skb_copy_bits(skb, skb->len - alen - 2, nexthdr, 2))
-		BUG();
+	ret = skb_copy_bits(skb, skb->len - alen - 2, nexthdr, 2);
+	BUG_ON(ret);
 
 	ret = -EINVAL;
 	padlen = nexthdr[0];