bcachefs: Fix incorrect error handling found_btree_node_is_readable()

error handling here is slightly odd, which is why we were accidently
calling evict() on an error pointer
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_node_scan.c

@@ -72,10 +72,11 @@ static bool found_btree_node_is_readable(struct btree_trans *trans,
 
 	struct btree *b = bch2_btree_node_get_noiter(trans, &k.k, f->btree_id, f->level, false);
 	bool ret = !IS_ERR_OR_NULL(b);
-	if (ret) {
+	if (!ret)
+		return ret;
+
 	f->sectors_written = b->written;
 	six_unlock_read(&b->c.lock);
-	}
 
 	/*
 	 * We might update this node's range; if that happens, we need the node