Let's say you make the first call dfs2(u) and then your condition : if(adj[u].size() != cur_sz — 1) is not true then you will always return true.

bool dfs2(int u)
{
	if(vis[u])
		return false;
	vis[u] = 1;
	if(adj[u].size() != cur_sz - 1)
		return false;
	for(int v : adj[u])
		if(!vis[v]) // you need to return what dfs2(v) gives u
			dfs2(v);

	return true; 
}

But thatshould not be the answer because imagine you have and edge (u,v) and then you call dfs2(v) and for v the condition if(adj[v].size() != cur_sz — 1) is true, then ur answer should be false. But you are always returing true because the call dfs(u) does that.

Hope it helps :)