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>
c923f6ac3b