File size is not persisted after opening the file with O_TRUNC flag and creating hard link if system crashes

[Vyacheslav Kovalevsky]

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

Hello, there seems to be an issue with btrfs crash behavior:

1. Create an empty file in a directory.
2. Fill file with data (e.g. truncate) and sync the file.
3. Open file with O_TRUNC flag (should set size to 0) and sync the file.
4. Create a new hard link to the file in the same directory.
5. Sync the directory.

After system crash the file will have the old size (>0) even though the 
file was synced after truncating (O_TRUNC).


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

Linux version 6.19.2


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;
  int file_fd;
  int dir_fd;

  status = creat("file1", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
  printf("CREAT: %d\n", status);

  status = truncate("file1", 1000);
  printf("TRUNCATE: %d\n", status);

  sync();

  status = open("file1", O_RDWR | O_TRUNC);
  printf("OPEN: %d\n", status);
  file_fd = status;

  status = fsync(file_fd);
  printf("FSYNC: %d\n", status);

  status = open(".", O_RDONLY | O_DIRECTORY);
  printf("OPEN: %d\n", status);
  dir_fd = status;

  status = link("file1", "file2");
  printf("LINK: %d\n", status);

  status = fsync(dir_fd);
  printf("FSYNC: %d\n", status);
}
// file size is 1000 instead of 0
```

Steps:

1. Create and mount new btrfs 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 size is 1000 instead of 0.