[IPV6] ROUTE: Assign rt6i_idev for ip6_{prohibit,blk_hole}_entry.

I think this is less critical, but is also suitable for -stable
release.

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