ping: correct the fib-index used for the reply

ping: correct the fib-index used for the reply

Type: fix

if original packet was to the link local, then the
fib index in the buffer is that of the LL table, we can't use that
to foward the response if the new destination
is global, so reset to the fib index of the link.
In other case, the fib index we need has been written
to the buffer already.

Add a test for IPv6 ping in an MPLS-VPN where int inout interface is
not the the same VRF as the response should be sent.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I18a232d90ddd3ef051a52476c5d861c87060e76f