btrfs: list_entry can't return NULL

No need to test the result, we can't get a
null pointer from list_entry()
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/disk-io.c

@@ -3846,8 +3846,6 @@ int btrfs_cleanup_transaction(struct btrfs_root *root)
 
 	while (!list_empty(&list)) {
 		t = list_entry(list.next, struct btrfs_transaction, list);
-		if (!t)
-			break;
 
 		btrfs_destroy_ordered_operations(root);