selftests: filesystems: fix warn_unused_result build warnings

Add return value checks for read & write calls in test_listmount_ns
function. This patch resolves below compilation warnings:

```
statmount_test_ns.c: In function ‘test_listmount_ns’:

statmount_test_ns.c:322:17: warning: ignoring return value of ‘write’
declared with attribute ‘warn_unused_result’ [-Wunused-result]

statmount_test_ns.c:323:17: warning: ignoring return value of ‘read’
declared with attribute ‘warn_unused_result’ [-Wunused-result]
```

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>