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>