drm/xe: Do not run GPU page fault handler on a closed VM

Closing a VM removes page table memory thus we shouldn't touch page
tables when a VM is closed. Do not run the GPU page fault handler once
the VM is closed to avoid touching page tables.

Signed-off-by: Matthew Brost <[e-mail]>
Reviewed-by: Himal Prasad Ghimiray <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/xe/xe_gt_pagefault.c

···
 	 * TODO: Change to read lock? Using write lock for simplicity.
 	 */
 	down_write(&vm->lock);
+
+	if (xe_vm_is_closed(vm)) {
+		err = -ENOENT;
+		goto unlock_vm;
+	}
+
 	vma = lookup_vma(vm, pf->page_addr);
 	if (!vma) {
 		err = -EINVAL;