[IPV6] ROUTE: Fix dst reference counting in ip6_pol_route_lookup().

In ip6_pol_route_lookup(), when we finish backtracking at the
top-level root entry, we need to hold it.

Bug noticed by Mitsuru Chinen <CHINEN@jp.ibm.com>.

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