[IPV6] ROUTE: No longer handle ::/0 specially.

We do not need to handle ::/0 routes specially any longer.
This should fix BUG #8349.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Yuji Sekiya <sekiya@wide.ad.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>