veth: add vlan features

The veth device doesn't provide the vlan features,
so TSO for example is disabled and that causes
performance issues when using tagged traffic.

The test topology looks like this:

    br0                     br1
  /   \                  /     \
vnet  veth0.10 ----- veth1.10   vnet
VM                               VM

The netperf results with current veth driver:
MIGRATED TCP STREAM TEST from 192.168.1.1 ()
port 0 AF_INET to 192.168.1.2 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.01    2210.22

Now after applying the proposed patch:
MIGRATED TCP STREAM TEST from 192.168.1.1 ()
port 0 AF_INET to 192.168.1.2 () port 0 AF_INET
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.00    13067.47

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>