bcachefs: Add support for FS_IOC_GETFSSYSFSPATH

bcachefs: Add support for FS_IOC_GETFSSYSFSPATH

[TEST]:
```
$ cat ioctl_getsysfspath.c
 #include <stdio.h>
 #include <stdlib.h>
 #include <fcntl.h>
 #include <sys/ioctl.h>
 #include <linux/fs.h>
 #include <unistd.h>

 int main(int argc, char *argv[]) {
     int fd;
     struct fs_sysfs_path sysfs_path = {};

     if (argc != 2) {
         fprintf(stderr, "Usage: %s <path_to_file_or_directory>\n", argv[0]);
         exit(EXIT_FAILURE);
     }

     fd = open(argv[1], O_RDONLY);
     if (fd == -1) {
         perror("open");
         exit(EXIT_FAILURE);
     }

     if (ioctl(fd, FS_IOC_GETFSSYSFSPATH, &sysfs_path) == -1) {
         perror("ioctl FS_IOC_GETFSSYSFSPATH");
         close(fd);
         exit(EXIT_FAILURE);
     }

     printf("FS_IOC_GETFSSYSFSPATH: %s\n", sysfs_path.name);
     close(fd);
     return 0;
 }

$ gcc ioctl_getsysfspath.c
$ sudo bcachefs format /dev/sda
$ sudo mount.bcachefs /dev/sda /mnt
$ sudo ./a.out /mnt
  FS_IOC_GETFSSYSFSPATH: bcachefs/c380b4ab-fbb6-41d2-b805-7a89cae9cadb
```

Original patch link:
[1]: https://lore.kernel.org/all/20240207025624.1019754-8-kent.overstreet@linux.dev/

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Youling Tang <youling.tang@linux.dev>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>