Claude review: mm: Wire up order in shrink_control

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

Patch Review

**Subject:** `[PATCH v4 1/6] mm: Wire up order in shrink_control`

This patch adds an `s8 order` field to `struct shrink_control` and threads the allocation order through `shrink_slab()` from `scan_control.order` to shrinkers.

**Looks correct.** The field is added cleanly:
```c
+	/* Allocation order we are currently trying to fulfil. */
+	s8 order;
```

The `shrink_slab` signature change is straightforward, threading `order` from `sc->order` at call sites in vmscan.c:
```c
-	shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);
+	shrink_slab(sc->gfp_mask, pgdat->node_id, sc->order, memcg,
+		    sc->priority);
```

The `drop_slab_node()` call correctly passes `0` as order since it's not driven by a specific allocation.

One minor nit: the `s8` type is a good match since `scan_control.order` is already `s8`, but it would be worth a brief justification in the commit message for why `s8` over `int` (consistency with scan_control). This is cosmetic.

**No functional issues. This patch is fine.**

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