For consistency with other L3 tunnel devices, reset the mac_header
pointer after decapsulation. This makes the mac_header 0 bytes long,
thus making it clear that this skb has no mac_header.

Compile tested only.
Signed-off-by: NGuillaume Nault <gnault@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>