io/ioutil: fix locking of the tempfile random state

Fixes #10451

Change-Id: I15ed777e9a4a4e1ba1178d07fcbe0ab4bf0d9e50
Reviewed-on: https://go-review.googlesource.com/8936Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/io/ioutil/tempfile.go

@@ -55,7 +55,9 @@ func TempFile(dir, prefix string) (f *os.File, err error) {
 		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
 		if os.IsExist(err) {
 			if nconflict++; nconflict > 10 {
+				randmu.Lock()
 				rand = reseed()
+				randmu.Unlock()
 			}
 			continue
 		}
@@ -82,7 +84,9 @@ func TempDir(dir, prefix string) (name string, err error) {
 		err = os.Mkdir(try, 0700)
 		if os.IsExist(err) {
 			if nconflict++; nconflict > 10 {
+				randmu.Lock()
 				rand = reseed()
+				randmu.Unlock()
 			}
 			continue
 		}