mpls: allow routes on ip6gre devices

Summary:

This appears to be necessary and sufficient change to enable `MPLS` on
`ip6gre` tunnels (RFC4023).

This diff allows IP6GRE devices to be recognized by MPLS kernel module
and hence user can configure interface to accept packets with mpls
headers as well setup mpls routes on them.

Test Plan:

Test plan consists of multiple containers connected via GRE-V6 tunnel.
Then carrying out testing steps as below.

- Carry out necessary sysctl settings on all containers

```
sysctl -w net.mpls.platform_labels=65536
sysctl -w net.mpls.ip_ttl_propagate=1
sysctl -w net.mpls.conf.lo.input=1
```

- Establish IP6GRE tunnels

```
ip -6 tunnel add name if_1_2_1 mode ip6gre \
  local 2401:db00:21:6048:feed:0::1 \
  remote 2401:db00:21:6048:feed:0::2 key 1
ip link set dev if_1_2_1 up
sysctl -w net.mpls.conf.if_1_2_1.input=1
ip -4 addr add 169.254.0.2/31 dev if_1_2_1 scope link

ip -6 tunnel add name if_1_3_1 mode ip6gre \
  local 2401:db00:21:6048:feed:0::1 \
  remote 2401:db00:21:6048:feed:0::3 key 1
ip link set dev if_1_3_1 up
sysctl -w net.mpls.conf.if_1_3_1.input=1
ip -4 addr add 169.254.0.4/31 dev if_1_3_1 scope link
```

- Install MPLS encap rules on node-1 towards node-2

```
ip route add 192.168.0.11/32 nexthop encap mpls 32/64 \
  via inet 169.254.0.3 dev if_1_2_1
```

- Install MPLS forwarding rules on node-2 and node-3
```
// node2
ip -f mpls route add 32 via inet 169.254.0.7 dev if_2_4_1

// node3
ip -f mpls route add 64 via inet 169.254.0.12 dev if_4_3_1
```

- Ping 192.168.0.11 (node4) from 192.168.0.1 (node1) (where routing
  towards 192.168.0.1 is via IP route directly towards node1 from node4)
```
ping 192.168.0.11
```

- tcpdump on interface to capture ping packets wrapped within MPLS
  header which inturn wrapped within IP6GRE header

```
16:43:41.121073 IP6
  2401:db00:21:6048:feed::1 > 2401:db00:21:6048:feed::2:
  DSTOPT GREv0, key=0x1, length 100:
  MPLS (label 32, exp 0, ttl 255) (label 64, exp 0, [S], ttl 255)
  IP 192.168.0.1 > 192.168.0.11:
  ICMP echo request, id 1208, seq 45, length 64

0x0000:  6000 2cdb 006c 3c3f 2401 db00 0021 6048  `.,..l<?$....!`H
0x0010:  feed 0000 0000 0001 2401 db00 0021 6048  ........$....!`H
0x0020:  feed 0000 0000 0002 2f00 0401 0401 0100  ......../.......
0x0030:  2000 8847 0000 0001 0002 00ff 0004 01ff  ...G............
0x0040:  4500 0054 3280 4000 ff01 c7cb c0a8 0001  E..T2.@.........
0x0050:  c0a8 000b 0800 a8d7 04b8 002d 2d3c a05b  ...........--<.[
0x0060:  0000 0000 bcd8 0100 0000 0000 1011 1213  ................
0x0070:  1415 1617 1819 1a1b 1c1d 1e1f 2021 2223  .............!"#
0x0080:  2425 2627 2829 2a2b 2c2d 2e2f 3031 3233  $%&'()*+,-./0123
0x0090:  3435 3637                                4567
```
Signed-off-by: Saif Hasan <has@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -1533,10 +1533,14 @@ static int mpls_dev_notify(struct notifier_block *this, unsigned long event,
 	unsigned int flags;
 
 	if (event == NETDEV_REGISTER) {
-		/* For now just support Ethernet, IPGRE, SIT and IPIP devices */
+
+		/* For now just support Ethernet, IPGRE, IP6GRE, SIT and
+		 * IPIP devices
+		 */
 		if (dev->type == ARPHRD_ETHER ||
 		    dev->type == ARPHRD_LOOPBACK ||
 		    dev->type == ARPHRD_IPGRE ||
+		    dev->type == ARPHRD_IP6GRE ||
 		    dev->type == ARPHRD_SIT ||
 		    dev->type == ARPHRD_TUNNEL) {
 			mdev = mpls_add_dev(dev);