A Mamba block-allocation miscount stalled entire vLLM servers.

PR #57050 fixes get_num_blocks_to_allocate miscalculating physical blocks for long requests loading prefix cache from external KV storage such as Mooncake, so the request is refused admission and sits in the waiting queue forever, stalling the whole system. Maintainers trace it to a likely regression from #53614. The reproduction needs 8 B300s serving Kimi-K3 with specific flags, narrow enough that most operators would have experienced it as an unexplained hang.