ip_tunnel: fix preempt warning in ip tunnel creation/updating

After the commit e09acddf ("ip_tunnel: replace dst_cache with generic
implementation"), a preemption debug warning is triggered on ip4
tunnels updating; the dst cache helper needs to be invoked in unpreemptible
context.

We don't need to load the cache on tunnel update, so this commit fixes
the warning replacing the load with a dst cache reset, which is
preempt safe.

Fixes: e09acddf ("ip_tunnel: replace dst_cache with generic implementation")
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -326,12 +326,12 @@ static int ip_tunnel_bind_dev(struct net_device *dev)
 
 		if (!IS_ERR(rt)) {
 			tdev = rt->dst.dev;
-			dst_cache_set_ip4(&tunnel->dst_cache, &rt->dst,
-					  fl4.saddr);
 			ip_rt_put(rt);
 		}
 		if (dev->type != ARPHRD_ETHER)
 			dev->flags |= IFF_POINTOPOINT;
+
+		dst_cache_reset(&tunnel->dst_cache);
 	}
 
 	if (!tdev && tunnel->parms.link)