drm/xe/bo: Block CPU faults to purgeable buffer objects

Block CPU page faults to buffer objects marked as purgeable (DONTNEED)
or already purged. Once a BO is marked DONTNEED, its contents can be
discarded by the kernel at any time, making access undefined behavior.
Return VM_FAULT_SIGBUS immediately to fail consistently instead of
allowing erratic behavior where access sometimes works (if not yet
purged) and sometimes fails (if purged).

For DONTNEED BOs:
- Block new CPU faults with SIGBUS to prevent undefined behavior.
- Existing CPU PTEs may still work until TLB flush, but new faults
  fail immediately.

For PURGED BOs:
- Backing store has been reclaimed, making CPU access invalid.
- Without this check, accessing existing mmap mappings would trigger
  xe_bo_fault_migrate() on freed backing store, causing kernel hangs
  or crashes.

The purgeable check is added to both CPU fault paths:
- Fastpath (xe_bo_cpu_fault_fastpath): Returns VM_FAULT_SIGBUS
  immediately under dma-resv lock, preventing attempts to
  migrate/validate DONTNEED/purged pages.
- Slowpath (xe_bo_cpu_fault): Returns -EFAULT under drm_exec lock,
  converted to VM_FAULT_SIGBUS.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260326130843.3545241-5-arvind.yadav@intel.com

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