net: ipv6: more places need LOOPBACK_IFINDEX for flowi6_iif

To properly match iif in ip rules we have to provide
LOOPBACK_IFINDEX in flowi6_iif, not 0. Some ip6mr_fib_lookup
and fib6_rule_lookup callers need such fix.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>