netfilter: don't set F_IFACE on ipv6 fib lookups

"fib" starts to behave strangely when an ipv6 default route is
added - the FIB lookup returns a route using 'oif' in this case.

This behaviour was inherited from ip6tables rpfilter so change
this as well.

Bugzilla: https://bugzilla.netfilter.org/show_bug.cgi?id=1221


Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>