Check exact slab length and capacity transitions in percpu_tcmalloc_fuzz.

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Check exact slab length and capacity transitions in percpu_tcmalloc_fuzz.

Snapshot Length and Capacity around every operation. On a cached slab of a
running CPU, Push/PushBatch must take exactly min(n, capacity - length)
objects and Pop/PopBatch must yield exactly min(n, length); Length always
moves by the reported count. Grow and GrowOtherCache raise capacity by
exactly min(len, max_capacity - capacity) (Grow may also abort with 0),
ShrinkOtherCache lowers it by min(len, capacity) and returns only the objects
displaced beyond the unused capacity, and Drain hands back every object with
the prior capacity and leaves capacity at zero rather than merely bounded.