net: vrf: Fix ping failed when vrf mtu is set to 0

When the mtu of a vrf device is set to 0, it would cause ping
failed. So I think we should limit vrf mtu in a reasonable range
to solve this problem. I set dev->min_mtu to IPV6_MIN_MTU, so it
will works for both ipv4 and ipv6. And if dev->max_mtu still be 0
can be confusing, so I set dev->max_mtu to ETH_MAX_MTU.

Here is the reproduce step:

1.Config vrf interface and set mtu to 0:
3: enp4s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel
master vrf1 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:9e:dd:c1 brd ff:ff:ff:ff:ff:ff

2.Ping peer:
3: enp4s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel
master vrf1 state UP group default qlen 1000
    link/ether 52:54:00:9e:dd:c1 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.1/16 scope global enp4s0
       valid_lft forever preferred_lft forever
connect: Network is unreachable

3.Set mtu to default value, ping works:
PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
64 bytes from 10.0.0.2: icmp_seq=1 ttl=64 time=1.88 ms

Fixes: ad49bc63 ("net: vrf: remove MTU limits for vrf device")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vrf.c

@@ -1275,8 +1275,12 @@ static void vrf_setup(struct net_device *dev)
 	dev->priv_flags |= IFF_NO_QUEUE;
 	dev->priv_flags |= IFF_NO_RX_HANDLER;
 
-	dev->min_mtu = 0;
-	dev->max_mtu = 0;
+	/* VRF devices do not care about MTU, but if the MTU is set
+	 * too low then the ipv4 and ipv6 protocols are disabled
+	 * which breaks networking.
+	 */
+	dev->min_mtu = IPV6_MIN_MTU;
+	dev->max_mtu = ETH_MAX_MTU;
 }
 
 static int vrf_validate(struct nlattr *tb[], struct nlattr *data[],