maple_tree: refine mas_store_root() on storing NULL

maple_tree: refine mas_store_root() on storing NULL

Currently, when storing NULL on mas_store_root(), the behavior could be
improved.

For example possible cases are:

  * store NULL at any range result a new node
  * store NULL at range [m, n] where m > 0 to a single entry tree result
    a new node with range [m, n] set to NULL
  * store NULL at range [m, n] where m > 0 to an empty tree result
    consecutive NULL slot
  * it allows for multiple NULL entries by expanding root
    to store NULLs to an empty tree

This patch tries to improve in:

  * memory efficient by setting to empty tree instead of using a node
  * remove the possibility of consecutive NULL slot which will prohibit
    extended null in later operation

Link: https://lkml.kernel.org/r/20241031231627.14316-5-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>