ipv6: allow ping to link-local address in VRF

If link-local packets are marked as enslaved to a VRF, then to allow
ping to the link-local from a vrf, the error handling for IPV6_PKTINFO
needs to be relaxed to also allow the pkt ipi6_ifindex to be that of a
slave device to the vrf.

Note that the real device also needs to be retrieved in icmp6_iif()
to set the ipv6 flow oif to this for icmp echo reply handling. The
recent commit 24b711ed ("net/ipv6: Fix linklocal to global address
with VRF") takes care of this, so the sdif does not need checking here.

This fix makes ping to link-local consistent with that to global
addresses, in that this can now be done from within the same VRF that
the address is in.

Signed-off-by: Mike Manning <mmanning@vyatta.att-mail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Tested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>