ipv6: Process unicast packet with Router Alert by checking flag in skb.

Router Alert option is marked in skb.
Previously, IP6CB(skb)->ra was set to positive value for such packets.
Since commit dd3332bf ("ipv6: Store Router Alert option in IP6CB
directly."), IP6SKB_ROUTERALERT is set in IP6CB(skb)->flags, and
the value of Router Alert option (in network byte order) is set
to IP6CB(skb)->ra for such packets.

Multicast forwarding path uses that flag and value, but unicast
forwarding path does not use the flag and misuses IP6CB(skb)->ra
value.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>