File name is not persisted if opened with O_SYNC and O_TRUNC flags

[Vyacheslav Kovalevsky]

Detailed description
====================

Hello, there seems to be an issue with O_SYNC flag when used together 
with O_TRUNC on various file systems.
Opening a file with O_SYNC (or using fsync(fd)) should persist directory 
entry.
However, if O_SYNC is used together with O_TRUNC the file will be 
missing if system crashes.
According to POSIX this is OK, but most file systems provide stronger 
guarantees (would be actually nice to have a more recent documentation 
on this behavior).
This happens on Btrfs, ext4, XFS, F2FS and likely other file systems.


System info
===========

Linux version 6.19-rc7, also tested on 6.17


How to reproduce
================

```
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
  int status;

  status = creat("file", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
  printf("CREAT: %d\n", status);
  close(status);

  status = open("file", O_RDWR | O_TRUNC | O_SYNC);
  printf("OPEN: %d\n", status);
}
// after the crash `file` is missing
```

Steps:
1. Create and mount new file system in default configuration.
2. Change directory to root of the file system and run the compiled test.
3. Cause hard system crash (e.g. QEMU `system_reset` command).
4. Remount file system after crash.
5. Observe that file is missing.