bring _query_start_loc_to_chunk_indices_offsets back to test_mamba_ssm_ssd.py

[hl475]

Purpose

#24683 removed the helper function _query_start_loc_to_chunk_indices_offsets and breaks the tests/kernels/mamba/test_mamba_ssm_ssd.py (sample job https://buildkite.com/vllm/ci/builds/32891/steps/canvas?sid=019998c8-1682-4e2d-8161-55a772c16cde). This PR add the helper function back

Test Plan

Test Result

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