Per the comments at the head of fs_nacl.go, unexported methods expect
the fs mutex to have been taken by the caller.
This change brings Link and Rename into line with the other exported
functions wrt fs locking.

Fixes #22690

Change-Id: I46d08f7d227f23ff49bb0099d218214364a45e1a
Reviewed-on: https://go-review.googlesource.com/79295Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>