Claude review: mm: Add helper to convert HMM pfn to migrate pfn

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Minor issues.**

1. **New flags consume bits from the order field.** The `HMM_PFN_ORDER_SHIFT` moves from `BITS_PER_LONG - 11` to `BITS_PER_LONG - 13`, reducing the available order bits from 5 to 5 (no change, since bits 7 and 8 are now flags). However, the two new flags (`HMM_PFN_MIGRATE` at bit 7, `HMM_PFN_COMPOUND` at bit 8) sit between the existing flag bits and the order field. This is fine for correctness but the comment block should document the full bit layout.

2. **`HMM_PFN_COMPOUND` documentation typo:**
```
+ * HMM_PFN_COMPOUND   - The entry is represents a > 0 order page
```
Should be "The entry represents a > 0 order page" (remove "is").

3. **`migrate_hmm_range_setup()` is `EXPORT_SYMBOL` (not `_GPL`)**. Given that this is a core MM function operating on migrate_vma internals, `EXPORT_SYMBOL_GPL` would be more appropriate to match the convention of `migrate_vma_setup` and friends which are `EXPORT_SYMBOL`.

4. **`migrate_vma_unmap()` call from `migrate_hmm_range_setup()`**: This works because both are in `migrate_device.c`, but `migrate_vma_unmap()` is static. If `migrate_hmm_range_setup()` were ever moved, this would break. Worth noting but not blocking.

5. **`range->migrate` uses spaces for alignment** instead of tabs in the struct definition:
```
+	struct migrate_vma      *migrate;
```
Should use a tab for consistency with the other fields.

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