drm/xe: Fix ref counting leak on page fault

If a page fault occurs on VM not in fault a ref can be leaked. Fix this.

Fixes: dd08ebf6c3 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240301041036.238471-1-matthew.brost@intel.com

drivers/gpu/drm/xe/xe_gt_pagefault.c

@@ -146,10 +146,12 @@ static int handle_pagefault(struct xe_gt *gt, struct pagefault *pf)
 	/* ASID to VM */
 	mutex_lock(&xe->usm.lock);
 	vm = xa_load(&xe->usm.asid_to_vm, pf->asid);
-	if (vm)
+	if (vm && xe_vm_in_fault_mode(vm))
 		xe_vm_get(vm);
+	else
+		vm = NULL;
 	mutex_unlock(&xe->usm.lock);
-	if (!vm || !xe_vm_in_fault_mode(vm))
+	if (!vm)
 		return -EINVAL;
 
 retry_userptr: