smb/client: fix incorrect nlink returned by fstat()

smb/client: fix incorrect nlink returned by fstat()

Reproducer:

  1. mount -t cifs //${server_ip}/export /mnt
  2. touch /mnt/file1; ln /mnt/file1 /mnt/file2; ln /mnt/file1 /mnt/file3
  3. C program: int fd = open("/mnt/file1", O_RDONLY);
  4. C program: struct stat stbuf; fstat(fd, &stbuf);
                stbuf.st_nlink is always 1, should be 3

Setting `unknown_nlink` to true in `SMB2_open()` triggers the
`CIFS_FATTR_UNKNOWN_NLINK` flag in `cifs_open_info_to_fattr()`,
which safely preserves the existing i_nlink in
`cifs_nlink_fattr_to_inode()`.

See the detailed procedure below:

  path_openat
    open_last_lookups
      lookup_open
        atomic_open
          cifs_atomic_open // dir->i_op->atomic_open
            cifs_lookup
              cifs_get_inode_info
                cifs_get_fattr
                  smb2_query_path_info // server->ops->query_path_info
                    smb2_compound_op
                      SMB2_open_init
                      case SMB2_OP_QUERY_INFO
                      SMB2_query_info_init(FILE_ALL_INFORMATION,)
                  cifs_open_info_to_fattr
                    fattr->cf_nlink = le32_to_cpu(info->NumberOfLinks)
                update_inode_info
                  cifs_iget
                    cifs_fattr_to_inode
                      cifs_nlink_fattr_to_inode
                        set_nlink(inode, fattr->cf_nlink)
    do_open
      vfs_open
        do_dentry_open
          cifs_open
            cifs_nt_open
              smb2_open_file // server->ops->open
                SMB2_open
                  buf->unknown_nlink = true
              cifs_get_inode_info
                cifs_get_fattr
                  cifs_open_info_to_fattr
                    if (data->unknown_nlink) // true
                    fattr->cf_flags |= CIFS_FATTR_UNKNOWN_NLINK
                update_inode_info
                  cifs_fattr_to_inode
                    cifs_nlink_fattr_to_inode
                      if (fattr->cf_flags & CIFS_FATTR_UNKNOWN_NLINK) // true
                      return // do not modify nlink

Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>