fix: do not ignore `ftruncate64()` return value


```
[tasks/bdev/src/bdev_runtime.cc:110](https://github.com/HDFGroup/hermes/blob/master/tasks/bdev/src/bdev_runtime.cc#L110):16: warning: ignoring return value of ‘int ftruncate64(int, __off64_t)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
```

Ref: https://my.cdash.org/viewBuildError.php?type=1&buildid=3121281