`ftruncate` resets file size to zero for the descriptor

[Slava0135]

Also see #78, #79.

Our fuzzer DIFFuzzer (source is closed at the moment) found this bug on commit 1a4d23203df14e84e910cdff48470c430f9560e4 (tag: v2.7.11)

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

int main() {
  int fd = creat("1", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
  close(fd);

  fd = open("1", O_RDWR);
  int truncated = ftruncate(fd, 64);
  printf("FTRUNCATE %d(%s)\n", truncated, strerror(errno));

  int position = lseek(fd, 0, SEEK_CUR);
  printf("LSEEK %d(%s)\n", position, strerror(errno));

  char buffer[1000] = {0};
  int nread = read(fd, buffer, 100);
  printf("READ %d(%s)\n", nread, strerror(errno));

  int size = lseek(fd, 0, SEEK_END);
  printf("LSEEK %d(%s)\n", size, strerror(errno));
}

// ::Expected::
// FTRUNCATE 0(Success)
// LSEEK 0(Success)
// READ 64(Success)
// LSEEK 64(Success)

// ::Actual::
// FTRUNCATE 0(Success)
// LSEEK 0(Success)
// READ 0(Success)
// LSEEK 0(Success)

ftruncate resets file size to zero for the descriptor.
However, file size was actually changed, if using stat:

$ stat 1
  File: 1
  Size: 64              Blocks: 0          IO Block: 4096   regular file
...

System info:

Ubuntu 22.04.5 LTS
Kernel 5.15.178
gcc 11.4.0