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.
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