File size checks fails and I haven't figured out why, but next I'd look into how `posix_fallocate` works on alpine linux.

Log:

```
$ cat /etc/alpine-release 
3.9.2

$ git log --oneline -1
e45b7f20e Merge #15618: refactor: Remove unused function

$ ./src/test/test_bitcoin -t flatfile_tests
Running 4 test cases...
Version using posix_fallocate
test/flatfile_tests.cpp(94): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 0))) == 100 has failed [0 != 100]
test/flatfile_tests.cpp(98): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 100 has failed [0 != 100]
Version using posix_fallocate
test/flatfile_tests.cpp(102): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 200 has failed [0 != 200]
Version using posix_fallocate
test/flatfile_tests.cpp(116): error: in "flatfile_tests/flatfile_flush": check fs::file_size(seq.FileName(FlatFilePos(0, 1))) == 100 has failed [0 != 100]

*** 4 failures are detected in the test module "Bitcoin Core Test Suite"
```
{
    std::lock_guard<std::mutex> lock(cs_dir_locks);
    dir_locks.erase((directory / lockfile_name).string());
}
void ReleaseDirectoryLocks()
{
    std::lock_guard<std::mutex> ulock(cs_dir_locks);
    dir_locks.clear();
}
bool DirIsWritable(const fs::path& directory)
{
    fs::path tmpFile = directory / fs::unique_path();
    FILE* file = fsbridge::fopen(tmpFile, "a");
    if (!file) return false;
    fclose(file);
    remove(tmpFile);
    return true;
}
bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes)