syscall: add missing fs locking in Link, Rename on nacl

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>

src/syscall/fs_nacl.go

@@ -625,6 +625,8 @@ func UtimesNano(path string, ts []Timespec) error {
 
 func Link(path, link string) error {
 	fsinit()
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
 	ip, _, err := fs.namei(path, false)
 	if err != nil {
 		return err
@@ -646,6 +648,8 @@ func Link(path, link string) error {
 
 func Rename(from, to string) error {
 	fsinit()
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
 	fdp, felem, err := fs.namei(from, true)
 	if err != nil {
 		return err