bpo-45521: Fix a bug in the obmalloc radix tree code. (GH-29051) (GH-29122)

MAP_BOT_LENGTH was incorrectly used to compute MAP_TOP_MASK instead of
MAP_TOP_LENGTH. On 64-bit machines, the error causes the tree to hold
46-bits of virtual addresses, rather than the intended 48-bits.
(cherry picked from commit 311910b31a4bd94dc79298388b7cb65ca5546438)