ospf6d: Fix memory leaks

ospf6d: Fix memory leaks

Free route node upon asbr redistribute route cleanup from
external_id_table route tale.
Free route node when route_remove is called and
node->info is set to null.
Decrement route node lock in route_lookup api as it
is incremented as part of node_lookup api.
use local variable for nexthop vs. malloc in zebra parse
routine.

two of the memory leaks related to nexthops per route were not freed.
two of the memory leak detected per frr service restart

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>