vxlan: Protect against tunnel config where source is not local

vxlan: Protect against tunnel config where source is not local

Type: fix

If a tunnel's source is not local then post encap VPP will attempt to
receive (via ip4-local) that packet, things go wrong from there.
The fix is when stacking the encap forwarding don't accept a receive
DPO. This approach is taken, rather than rejecting bad tunnels, because
the 'local-ness' of the tunnel's source can change and we can't reject
tunnels that were once correctly configured but are no longer.
the user will quickly discover their mistake as traffic won't pass.

Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: I46198422e321606e8baba003112e978a526b4c2f