maple_tree: update mas_preallocate() testing

Since the mas_preallocate() calculation has been updated to be more
precise, the testing must also be updated to check for what is expected.

Link: https://lkml.kernel.org/r/20230724183157.3939892-13-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>