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>