tuntap: initialize vlan_features

The vlan_features was zero which prevents vlan GSO packets to be transmitted to
userspace. This is suboptimal so enable this by initialize vlan_features for
tuntap.

Netperf shows better performance of guest receiving since vlan TSO works for
tuntap:

before:
netperf -H 192.168.5.4
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.5.4 ()
port 0 AF_INET : demo
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.01    2786.67

after:
netperf -H 192.168.5.4
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.5.4 ()
port 0 AF_INET : demo
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.00    8085.49
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1656,6 +1656,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 		dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
 			TUN_USER_FEATURES;
 		dev->features = dev->hw_features;
+		dev->vlan_features = dev->features;
 
 		INIT_LIST_HEAD(&tun->disabled);
 		err = tun_attach(tun, file);