drm/i915/ttm: only fault WILLNEED objects

Don't attempt to fault and re-populate purged objects. By some fluke
this passes the dontneed-after-mmap IGT, but for the wrong reasons.

Fixes: cf3e3e8 ("drm/i915: Use ttm mmap handling for ttm bo's.")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220106174910.280616-2-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -881,6 +881,11 @@ static vm_fault_t vm_fault_ttm(struct vm_fault *vmf)
 	if (ret)
 		return ret;
 
+	if (obj->mm.madv != I915_MADV_WILLNEED) {
+		dma_resv_unlock(bo->base.resv);
+		return VM_FAULT_SIGBUS;
+	}
+
 	if (drm_dev_enter(dev, &idx)) {
 		ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
 					       TTM_BO_VM_NUM_PREFAULT);