ipv6: Remove some pointless conditionals before kfree_skb()

Remove some pointless conditionals before kfree_skb().

The semantic match that finds the problem is as follows:
(http://www.emn.fr/x-info/coccinelle/

)

// <smpl>
@@
expression E;
@@
- if (E)
- 	kfree_skb(E);
+ kfree_skb(E);
// </smpl>

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>