Claude review: drm/pagemap: Unlock and put folios when possible

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

Patch Review

Clean conversion from page-level to folio-level unlock/put. The logic change from `++i` increment with `continue` to bottom-of-loop `i += NR_PAGES(order)` with `goto next` is correct.

One subtlety worth verifying: when `!migrate_pfn[i]`, `order` remains 0 so `NR_PAGES(0) = 1`, correctly advancing by one entry. This is correct.

```c
+		if (!migrate_pfn[i])
+			goto next;
...
+next:
+		i += NR_PAGES(order);
```

The `migrate_pfn[i] = 0` assignment after the folio put is only done for the head entry. For a multi-page folio, the tail entries in `migrate_pfn[]` are not zeroed. This matches the existing behavior since the loop now skips over tail entries via `NR_PAGES(order)`, but callers that later iterate `migrate_pfn[]` linearly (without folio awareness) could see stale non-zero entries for tail pages. This should be fine since callers are being converted to folio-aware iteration in the rest of the series.

**Verdict: Looks good.**

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