os: Plan 9: allocate space for a string in Rename

The Name field of the stat structure is variable length
and the marshalling code in package syscall requires
a buf long enough to contain the Name as well as the
static data. This change makes sure that the buffer in
os.Rename is allocated with the appropriate length.

R=rsc, rminnich, ality, r
CC=golang-dev
https://golang.org/cl/7453044

src/pkg/os/file_plan9.go

@@ -308,7 +308,7 @@ func Rename(oldname, newname string) error {
 	d.Null()
 	d.Name = newname
 
-	var buf [syscall.STATFIXLEN]byte
+	buf := make([]byte, syscall.STATFIXLEN+len(d.Name))
 	n, err := d.Marshal(buf[:])
 	if err != nil {
 		return &PathError{"rename", oldname, err}