veth: extend device features

veth is lacking most modern facilities, like SG, checksums, TSO.

It makes sense to extend dev->features to get them, or GRO aggregation
is defeated by a forced segmentation.

Reported-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>