net: ipv6: Add RT6_LOOKUP_F_IFACE flag if oif is set

Wolfgang reported that IPv6 stack is ignoring oif in output route lookups:

With ipv6, ip -6 route get always returns the specific route.

$ ip -6 r
2001:db8:e2::1 dev enp2s0 proto kernel metric 256
2001:db8:e2::/64 dev enp2s0 metric 1024
2001:db8:e3::1 dev enp3s0 proto kernel metric 256
2001:db8:e3::/64 dev enp3s0 metric 1024
fe80::/64 dev enp3s0 proto kernel metric 256
default via 2001:db8:e3::255 dev enp3s0 metric 1024

$ ip -6 r get 2001:db8:e2::100
2001:db8:e2::100 from :: dev enp2s0 src 2001:db8:e3::1 metric 0
cache

$ ip -6 r get 2001:db8:e2::100 oif enp3s0
2001:db8:e2::100 from :: dev enp2s0 src 2001:db8:e3::1 metric 0
cache

The stack does consider the oif but a mismatch in rt6_device_match is not
considered fatal because RT6_LOOKUP_F_IFACE is not set in the flags.

Cc: Wolfgang Nothdurft <[e-mail]>
Signed-off-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/route.c

···
 
 	fl6->flowi6_iif = LOOPBACK_IFINDEX;
 
-	if ((sk && sk->sk_bound_dev_if) || rt6_need_strict(&fl6->daddr))
+	if ((sk && sk->sk_bound_dev_if) || rt6_need_strict(&fl6->daddr) ||
+	    fl6->flowi6_oif)
 		flags |= RT6_LOOKUP_F_IFACE;
 
 	if (!ipv6_addr_any(&fl6->saddr))