ipv4: Add a missing rcu_assign_pointer() in routing cache.

rt_intern_hash() is doing an update of a RCU guarded hash chain
without using rcu_assign_pointer() or equivalent barrier.
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1109,7 +1109,12 @@ static int rt_intern_hash(unsigned hash, struct rtable *rt, struct rtable **rp)
 		printk("\n");
 	}
 #endif
-	rt_hash_table[hash].chain = rt;
+	/*
+	 * Since lookup is lockfree, we must make sure
+	 * previous writes to rt are comitted to memory
+	 * before making rt visible to other CPUS.
+	 */
+	rcu_assign_pointer(rt_hash_table[hash].chain, rt);
 	spin_unlock_bh(rt_hash_lock_addr(hash));
 	*rp = rt;
 	return 0;