Why do you think that's needed? Of course if majority of nodes cheated and said "sure, I've got that file" and send you random garbage instead so you have to retry, that would be bad. But if majority are running proper implementation, you don't really need to trust anyone.
IPFS is content-addressed. That means that it's trivial to discard tampered content. Lying majority, sure, will impact your content retrieval speed — but dealing with cheaters seems to be as simple as adding their addresses in some ignore list (which can itself be IPFS-shared, if needed).
I'm not sure of this blacklisting is implemented in current go-ipfs out js-ipfs codebase — but hash verification itself certainly is.
You need a reasonable number of nodes. Likely a large majority. Otherwise you'll just keep redownloading garbage data and discarding it. And likely doing it slower than bad nodes can leave/rejoin the network with new ips.
Why do you think that's needed? Of course if majority of nodes cheated and said "sure, I've got that file" and send you random garbage instead so you have to retry, that would be bad. But if majority are running proper implementation, you don't really need to trust anyone.