exfat: convert exfat_remove_entries() to use dentry cache

Before this conversion, in exfat_remove_entries(), to mark the
dentries in a dentry set as deleted, the sync times is equals
the dentry numbers if 'dirsync' or 'sync' is enabled.
That affects not only performance but also device life.

After this conversion, only needs to be synchronized once if
'dirsync' or 'sync' is enabled.

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>