bcachefs: Fix failure to relock in btree_node_get()

discovered by new trans->locked asserts
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.c

@@ -974,6 +974,10 @@ static struct btree *__bch2_btree_node_get(struct btree_trans *trans, struct btr
 
 		bch2_btree_node_wait_on_read(b);
 
+		ret = bch2_trans_relock(trans);
+		if (ret)
+			return ERR_PTR(ret);
+
 		/*
 		 * should_be_locked is not set on this path yet, so we need to
 		 * relock it specifically: