netfilter: Update nf_send_reset6 to consider L3 domain

nf_send_reset6 is not considering the L3 domain and lookups are sent
to the wrong table. For example consider the following output rule:

ip6tables -A OUTPUT -p tcp --dport 12345 -j REJECT --reject-with tcp-reset

using perf to analyze lookups via the fib6_table_lookup tracepoint shows:

swapper     0 [001]   248.787816: fib6:fib6_table_lookup: table 255 oif 0 iif 1 src 2100:1::3 dst 2100:1:
        ffffffff81439cdc perf_trace_fib6_table_lookup ([kernel.kallsyms])
        ffffffff814c1ce3 trace_fib6_table_lookup ([kernel.kallsyms])
        ffffffff814c3e89 ip6_pol_route ([kernel.kallsyms])
        ffffffff814c40d5 ip6_pol_route_output ([kernel.kallsyms])
        ffffffff814e7b6f fib6_rule_action ([kernel.kallsyms])
        ffffffff81437f60 fib_rules_lookup ([kernel.kallsyms])
        ffffffff814e7c79 fib6_rule_lookup ([kernel.kallsyms])
        ffffffff814c2541 ip6_route_output_flags ([kernel.kallsyms])
                     528 nf_send_reset6 ([nf_reject_ipv6])

The lookup is directed to table 255 rather than the table associated with
the device via the L3 domain. Update nf_send_reset6 to pull the L3 domain
from the dst currently attached to the skb.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv6/netfilter/nf_reject_ipv6.c

@@ -156,6 +156,7 @@ void nf_send_reset6(struct net *net, struct sk_buff *oldskb, int hook)
 	fl6.daddr = oip6h->saddr;
 	fl6.fl6_sport = otcph->dest;
 	fl6.fl6_dport = otcph->source;
+	fl6.flowi6_oif = l3mdev_master_ifindex(skb_dst(oldskb)->dev);
 	security_skb_classify_flow(oldskb, flowi6_to_flowi(&fl6));
 	dst = ip6_route_output(net, NULL, &fl6);
 	if (dst->error) {