gtp: reset mac_header after decap

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: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>