vmxnet3: use correct hdr reference when packet is encapsulated

'Commit dacce2be ("vmxnet3: add geneve and vxlan tunnel offload
support")' added support for encapsulation offload. However, while
preparing inner tso packet, it uses reference to outer ip headers.

This patch fixes this issue by using correct reference for inner
headers.

Fixes: dacce2be ("vmxnet3: add geneve and vxlan tunnel offload support")
Signed-off-by: Ronak Doshi <doshir@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>