Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

Issue uncovered by inspection.

What operating system and processor architecture are you using (go env)?

Go playground using GOOS=nacl

What did you do?

Per comments at the head of fs_nacl.go:

// Since there is no I/O (everything is in memory),
// the global lock mu protects the whole file system state,
// and that's okay.

and

// Except where indicated otherwise, unexported methods on fsys
// expect fs.mu to have been locked by the caller.

but Link and Rename call these methods without locking fs.

What did you expect to see?

fs calls should be serialized

What did you see instead?

These two are not.
Fix is trivial. I'm happy the needed changes