exfat: reuse __exfat_write_inode() to update directory entry

__exfat_write_inode() is used to update file and stream directory
entries, except for file->start_clu and stream->flags.

This commit moves update file->start_clu and stream->flags to
__exfat_write_inode() and reuse __exfat_write_inode() to update
directory entries.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>