maple_tree: preserve the tree attributes when destroying maple tree

When destroying maple tree, preserve its attributes and then turn it into
an empty tree.  This allows it to be reused without needing to be
reinitialized.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peng Zhang <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Mike Christie <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
(cherry picked from commit 8e50d32)

lib/maple_tree.c

@@ -6774,7 +6774,7 @@ void __mt_destroy(struct maple_tree *mt)
 	if (xa_is_node(root))
 		mte_destroy_walk(root, mt);
 
-	mt->ma_flags = 0;
+	mt->ma_flags = mt_attr(mt);
 }
 EXPORT_SYMBOL_GPL(__mt_destroy);