openvswitch: Reset upper layer protocol info on internal devices.

It's possible that packets that are sent on internal devices (from
the OVS perspective) have already traversed the local IP stack.
After they go through the internal device, they will again travel
through the IP stack which may get confused by the presence of
existing information in the skb. The problem can be observed
when switching between namespaces. This clears out that information
to avoid problems but deliberately leaves other metadata alone.
This is to provide maximum flexibility in chaining together OVS
and other Linux components.

Signed-off-by: Jesse Gross <jesse@nicira.com>