maple_tree: fix mt_destroy_walk() on root leaf node

maple_tree: fix mt_destroy_walk() on root leaf node

commit ea9b77f98d94c4d5c1bd1ac1db078f78b40e8bf5 upstream.

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/20250407231354.11771-1-richard.weiyang@gmail.com/
Link: https://lkml.kernel.org/r/20250624191841.64682-1-Liam.Howlett@oracle.com
Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>