exfat: add __exfat_get_dentry_set() helper

Since exfat_get_dentry_set() invokes the validate functions of
exfat_validate_entry(), it only supports getting a directory
entry set of an existing file, doesn't support getting an empty
entry set.

To remove the limitation, add this helper.

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: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>