ipv6: fix warning in xfrm6_mode_tunnel_input

Should not use assignment in conditional:
 warning: suggest parentheses around assignment used as truth value [-Wparentheses]

Problem introduced by:
commit 14bbd6a5
Author: Pravin B Shelar <pshelar@nicira.com>
Date:   Thu Feb 14 09:44:49 2013 +0000

    net: Add skb_unclone() helper function.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/xfrm6_mode_tunnel.c

@@ -69,7 +69,8 @@ static int xfrm6_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 	if (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
 		goto out;
 
-	if (err = skb_unclone(skb, GFP_ATOMIC))
+	err = skb_unclone(skb, GFP_ATOMIC);
+	if (err)
 		goto out;
 
 	if (x->props.flags & XFRM_STATE_DECAP_DSCP)