Claude review: drm/xe/xe_pagefault: Disallow writes to read-only VMAs

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

Patch Review

This is tagged as a `Fixes:` patch, which is good.

**Bug: access_type comparison doesn't mask out prefetch bit.** The `access_type` field packs the actual access type in bits [1:0] and a prefetch flag in bit 7. The check:

```c
if (xe_vma_read_only(vma) &&
    pf->consumer.access_type != XE_PAGEFAULT_ACCESS_TYPE_READ) {
```

compares the raw `access_type` value, so a READ prefetch (`0x80`) would be incorrectly rejected as non-read. The existing `xe_pagefault_access_is_atomic()` at line 139 of xe_pagefault.c already demonstrates the correct pattern — mask first:

```c
return (access_type & XE_PAGEFAULT_ACCESS_TYPE_MASK) == XE_PAGEFAULT_ACCESS_TYPE_ATOMIC;
```

The fix should be:
```c
if (xe_vma_read_only(vma) &&
    (pf->consumer.access_type & XE_PAGEFAULT_ACCESS_TYPE_MASK) != XE_PAGEFAULT_ACCESS_TYPE_READ) {
```

Notably, patch 4 adds `xe_to_user_access_type()` which correctly uses the mask, yet this patch doesn't. This is a real bug since prefetch faults on read-only VMAs would be wrongly rejected with `-EPERM` instead of being serviced.

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