UPSTREAM: exfat: convert exfat_find_empty_entry() to use dentry cache

Before this conversion, each dentry traversed needs to be read
from the storage device or page cache. There are at least 16
dentries in a sector. This will result in frequent page cache
searches.

After this conversion, if all directory entries in a sector are
used, the sector only needs to be read once.

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>
(cherry picked from commit af02c72d)

Change-Id: Ic0540a5f79b0326b3ab1c3cd9a87506a22792ced
Signed-off-by: dongliang.cui <dongliang.cui@unisoc.com>