ipv6: fix ecmp lookup when oif is specified

There is no reason to skip ECMP lookup when oif is specified, but this implies
to check oif given by user when selecting another route.
When the new route does not match oif requirement, we simply keep the initial
one.

Spotted-by: dingzhi <zhi.ding@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>