ip6: set local flag on outbound echo reply

Type: fix

When VPP generates an ICMP echo reply in response to an incoming
echo request to a local address, set VNET_BUFFER_F_LOCALLY_ORIGINATED
on the buffer. It will prevent ip6-rewrite from decrementing the hop
limit.

Outbound IPv4 echo replies also get this flag set.

Change-Id: Iaa229294eb158edb58cf1bf1b7a90da281321406
Signed-off-by: Matthew Smith <mgsmith@netgate.com>