net: dummy: add more features

While testing my SIT/GRO patch using netfilter TEE module and a dummy
device, I found some features were missing :

TSO IPv6, UFO, and encapsulated traffic.

ethtool -k dummy0 now gives :
...
tcp-segmentation-offload: on
	tx-tcp-segmentation: on
	tx-tcp-ecn-segmentation: on
	tx-tcp6-segmentation: on
udp-fragmentation-offload: on
...
tx-gre-segmentation: on
tx-ipip-segmentation: on
tx-sit-segmentation: on
tx-udp_tnl-segmentation: on

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>