maple_tree: clear up index and last setting in single entry tree

maple_tree: clear up index and last setting in single entry tree

When there is a single entry tree (range of 0-0 pointing to an entry),
then ensure the limit is either 0-0 or 1-oo, depending on where the user
walks.  Ensure the correct node setting as well; either MAS_ROOT or
MAS_NONE.

Link: https://lkml.kernel.org/r/20230518145544.1722059-33-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: David Binderman <dcb314@hotmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>