veth: set peer GSO values

When new veth is created, and GSO values have been configured
on one device, clone those values to the peer.

For example:
   # ip link add dev vm1 gso_max_size 65530 type veth peer name vm2

This should create vm1 <--> vm2 with both having GSO maximum
size set to 65530.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>