ipv4: remove redundant null pointer check before kfree_skb

kfree_skb has taken the null pointer into account. hence it is safe
to remove the redundant null pointer check before kfree_skb.
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_fragment.c

@@ -260,8 +260,7 @@ static void ip_expire(struct timer_list *t)
 	spin_unlock(&qp->q.lock);
 out_rcu_unlock:
 	rcu_read_unlock();
-	if (head)
-		kfree_skb(head);
+	kfree_skb(head);
 	ipq_put(qp);
 }