vmxnet3: use ext1 field to indicate encapsulated packet

Till vmxnet3 version 6, om field of transmit descriptor was used
to indicate encapsulated offload packet and msscof was used to
indirectly indicate TSO/CSO. From version 7 and later, ext1 field
will be used to indicate whether packet is encapsulated or not and
om fields will continue to indicate if the packet is TSO or CSO.

Signed-off-by: Ronak Doshi <doshir@vmware.com>
Acked-by: Guolin Yang <gyang@vmware.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>