bcachefs: Fix an assert in copygc thread shutdown path

We're not supposed to have nested (locked) btree_trans on the stack:
this means copygc shutdown needs to exit our btree_trans before exiting
the move_ctxt, which calls bch2_write().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/movinggc.c

@@ -334,8 +334,8 @@ static int bch2_copygc_thread(void *arg)
 		wake_up(&c->copygc_running_wq);
 	}
 
-	bch2_moving_ctxt_exit(&ctxt);
 	bch2_trans_exit(&trans);
+	bch2_moving_ctxt_exit(&ctxt);
 	free_fifo(&move_buckets);
 
 	return 0;