Fix DCP block alignment before HMA expansion

[pisceskkk]

Purpose

Fix DCP block selection when NIXL transfers use hybrid memory allocation (HMA), prefix caching, or an incomplete final DCP stripe.

The read path previously expanded logical block IDs into kernel physical IDs before calling _dcp_read_slice. This made the DCP phase calculation count physical HMA blocks instead of logical blocks. In addition, equal-DCP transfers did not skip blocks already cached locally, and heterogeneous DCP slices could retain non-overlapping allocation padding.

This PR:

• matches local and remote DCP positions using logical block IDs, then expands the matched IDs for HMA;
• preserves the existing closed-form _dcp_read_slice calculation;
• applies the prefix offset for equal DCP sizes;
• truncates local and remote slices to their overlapping block count;
• clarifies the global-position example in _dcp_read_slice;
• adds regression coverage for HMA ordering, prefix phase, and local/remote tail padding.

Test Result

9 passed, 14 warnings in 4.19s

git diff --check and the repository sign-off commit hook also pass.

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Essential Elements of an Effective PR Description Checklist

• The purpose and root cause are described.
• The test plan is provided.
• The test result is provided.
• No separate documentation update is required; the affected function documentation is updated inline.