ipv6: Extend the route lookups to low priority metrics.

We search only for routes with highest priority metric in
find_rr_leaf(). However if one of these routes is marked
as invalid, we may fail to find a route even if there is
a appropriate route with lower priority. Then we loose
connectivity until the garbage collector deletes the
invalid route. This typically happens if a host route
expires afer a pmtu event. Fix this by searching also
for routes with a lower priority metric.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>