Mark Andrews authored Dec 04, 2019 and Ondřej Surý committed Dec 09, 2019

In decrement_reference only test node->down if the tree lock
is held.  As node->down is not always tested in
decrement_reference we need to test that it is non NULL in
cleanup_dead_nodes prior to removing the node from the rbt
tree.  Additionally it is not always possible to aquire the
node lock and reactivate a node when adding parent nodes.
Reactivate such nodes in cleanup_dead_nodes if required.