flow: Extend struct flow to contain tunnel outer header.

flow: Extend struct flow to contain tunnel outer header.

Soon the kernel will begin supplying the information about the outer
IP header for tunneled packets and userspace will need to be able to
track it as part of the flow.  For the time being this is only used
internally by OVS and not exposed outwards to OpenFlow.  As a result,
this threads the information throughout userspace but simply stores
the existing tun_id in it.

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