exfat: convert exfat_add_entry() to use dentry cache

After this conversion, if "dirsync" or "sync" is enabled, the
number of synchronized dentries in exfat_add_entry() will change
from 2 to 1.

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>