ipv6: use the right ifindex when replying to icmpv6 from localhost

When replying to a ICMPv6 echo request that comes from localhost address
the right output ifindex is 1 (lo) and not rt6i_idev dev index. Use the
skb device ifindex instead. This fixes pinging to a local address from
localhost source address.

$ ping6 -I ::1 2001:1:1::2 -c 3
PING 2001:1:1::2 (2001:1:1::2) from ::1 : 56 data bytes
64 bytes from 2001:1:1::2: icmp_seq=1 ttl=64 time=0.037 ms
64 bytes from 2001:1:1::2: icmp_seq=2 ttl=64 time=0.069 ms
64 bytes from 2001:1:1::2: icmp_seq=3 ttl=64 time=0.122 ms

2001:1:1::2 ping statistics
3 packets transmitted, 3 received, 0% packet loss, time 2032ms
rtt min/avg/max/mdev = 0.037/0.076/0.122/0.035 ms

Fixes: 1b70d792cf67 ("ipv6: Use rt6i_idev index for echo replies to a local address")
Signed-off-by: Fernando Fernandez Mancera <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/icmp.c

···
 	fl6.daddr = ipv6_hdr(skb)->saddr;
 	if (saddr)
 		fl6.saddr = *saddr;
-	fl6.flowi6_oif = icmp6_iif(skb);
+	fl6.flowi6_oif = ipv6_addr_loopback(&fl6.daddr) ?
+			 skb->dev->ifindex :
+			 icmp6_iif(skb);
 	fl6.fl6_icmp_type = type;
 	fl6.flowi6_mark = mark;
 	fl6.flowi6_uid = sock_net_uid(net, NULL);