Claude review: drm/xe/vm: Add srcid to xe_vm_get_property_ioctl fault report

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

Patch Review

**UAPI change:** The `__u8 pad` in `struct xe_vm_fault` is renamed to `__u8 srcid`. Since `pad` was MBZ and occupied the same offset, this is ABI-compatible — old userspace that set pad=0 will just read srcid=0 (or whatever value is there), and old kernels receiving new userspace won't break because the pad check was on the `drm_xe_vm_get_property` struct, not on the `xe_vm_fault` struct itself.

Looking at `xe_vm.c:4200-4202`, the pad check is on `args->pad` (the `drm_xe_vm_get_property` struct's pad, not `xe_vm_fault`'s pad), so this is indeed unaffected — the cover letter's "Readd pad check" note correctly identifies that the `drm_xe_vm_get_property.pad` check should remain.

**Nit:** `xe_to_user_srcid()` is a trivial identity function:
```c
static u8 xe_to_user_srcid(u8 srcid)
{
	return srcid;
}
```
This follows the pattern of `xe_to_user_fault_type()` and `xe_to_user_fault_level()` which are also identity functions. I understand these exist as placeholders for potential future remapping (similar to the XXX comment about enum matching in `xe_guc_pagefault.c:74-76`), so this is consistent with the codebase style.

The `xe_vm_fault_entry` struct addition and the `xe_vm_add_fault_entry_pf` plumbing are both correct and straightforward.

No blocking issues with this patch.

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