Claude review: vfio/pci: Add mmap() attributes to DMABUF feature

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

Patch Review

Adds `VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR` for setting UC/WC memory attributes on exported DMABUFs.

**Bug -- wrong errno**:
```c
+			default:
+				ret = -ENOTSUPP;
```
`-ENOTSUPP` is `524` (kernel-internal, `EOPNOTSUPP` equivalent for in-kernel use only). For a UAPI-facing path that returns to userspace, this should be `-EOPNOTSUPP` (`95`). The docstring even says `ENOTSUPP` but userspace will see the raw `524` value which is non-standard. Fix:
```c
				ret = -EOPNOTSUPP;
```

**GET+SET combined semantics**: When both `VFIO_DEVICE_FEATURE_GET` and `VFIO_DEVICE_FEATURE_SET` are specified, the code applies SET first, then returns the *old* value via GET:
```c
+		uint32_t old_attr = priv->memattr;
+		if (flags & VFIO_DEVICE_FEATURE_SET) { ... }
+		db_attr.memattr = old_attr;
+	}
+	if (!ret && (flags & VFIO_DEVICE_FEATURE_GET)) {
+		if (copy_to_user(arg, &db_attr, sizeof(db_attr)))
```
This "swap" semantic (set new, return old) is unusual and should be documented in the UAPI header. Users would typically expect GET to return the current (post-SET) value. Consider whether this is the intended behavior or a bug.

**`memattr` initialization for BAR mmap DMABUFs**: In `vfio_pci_core_mmap_prep_dmabuf()`, `priv->memattr` is not explicitly set, relying on `kzalloc` zero-initialization to give `VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_NC = 0`. This is fine but fragile -- if the enum values ever change, this would break. The explicit `VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_NC` enum name is used in patch 9 for the explicit export path, so it would be consistent to also set it explicitly in the mmap path. (Low priority.)

**Naming inconsistency**: The enum uses `_NC` (noncached) but the UAPI docstring says "The default ... is NC." Meanwhile patch 7 uses `pgprot_noncached`. The cover letter and commit message say "UC" (uncached). These are the same thing in practice, but the UAPI should pick one term and stick with it. `UC` is more conventional for MMIO.

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