maple_tree: fix mt_destroy_walk() on root leaf node

On destroy, we should set each node dead. But current code miss this when
the maple tree has only the root node.

The reason is mt_destroy_walk() leverage mte_destroy_descend() to set node
dead, but this is skipped since the only root node is a leaf.

Fixes this by setting the node dead if it is a leaf.

Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Signed-off-by: Wei Yang <[e-mail]>
Signed-off-by: Liam R. Howlett <[e-mail]>
Reviewed-by: Dev Jain <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/maple_tree.c

···
 	struct maple_enode *start;
 
 	if (mte_is_leaf(enode)) {
+		mte_set_node_dead(enode);
 		node->type = mte_node_type(enode);
 		goto free_leaf;
 	}