vxlan: capture inner headers during encapsulation

Allow VXLAN to make use of Tx checksum offloading and Tx scatter-gather.
The advantage to these two changes is that it also allows the VXLAN to
make use of GSO.

Signed-off-by: Joseph Gasparakis <joseph.gasparakis@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>