net: ipv4: add check for l3slave for index returned in IP_PKTINFO

Similar to the loopback device, for packets sent through a VRF device
the index returned in ipi_ifindex needs to be the saved index in
rt_iif.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>