Fix off-by-one bug

This is a fix for https://github.com/docker/for-linux/issues/1012.

The code was not considering that C strings are NULL-terminated so
we need to leave one extra byte.

Without this fix, the testcase in https://github.com/docker/for-linux/issues/1012
fails with

```
Step 61/1001 : RUN echo 60 > 60
 ---> Running in dde85ac3b1e3
Removing intermediate container dde85ac3b1e3
 ---> 80a12a18a241
Step 62/1001 : RUN echo 61 > 61
error creating overlay mount to /23456789112345678921234/overlay2/d368abcc97d6c6ebcf23fa71225e2011d095295d5d8c9b31d6810bea748bdf07-init/merged: no such file or directory
```

with the output of `dmesg -T` as:

```
[Sat Dec 19 02:35:40 2020] overlayfs: failed to resolve '/23456789112345678921234/overlay2/89e435a1b24583c463abb73e8abfad8bf8a88312ef8253455390c5fa0a765517-init/wor': -2
```

with this fix, you get the expected:

```
Step 126/1001 : RUN echo 125 > 125
 ---> Running in 2f2e56da89e0
max depth exceeded
```

Signed-off-by: Oscar Bonilla <6f6231@gmail.com>

daemon/graphdriver/overlay2/overlay.go

@@ -573,14 +573,14 @@ func (d *Driver) Get(id, mountLabel string) (_ containerfs.ContainerFS, retErr e
 	// the page size. The mount syscall fails if the mount data cannot
 	// fit within a page and relative links make the mount data much
 	// smaller at the expense of requiring a fork exec to chroot.
-	if len(mountData) > pageSize {
+	if len(mountData) > pageSize-1 {
 		if readonly {
 			opts = indexOff + "lowerdir=" + path.Join(id, diffDirName) + ":" + string(lowers)
 		} else {
 			opts = indexOff + "lowerdir=" + string(lowers) + ",upperdir=" + path.Join(id, diffDirName) + ",workdir=" + path.Join(id, workDirName)
 		}
 		mountData = label.FormatMountLabel(opts, mountLabel)
-		if len(mountData) > pageSize {
+		if len(mountData) > pageSize-1 {
 			return nil, fmt.Errorf("cannot mount layer, mount label too large %d", len(mountData))
 		}