UPSTREAM: exfat: add exfat_get_empty_dentry_set() helper

This helper is used to lookup empty dentry set. If there are
no enough empty dentries at the input location, this helper will
return the number of dentries that need to be skipped for the
next lookup.

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 01da3a51)

Bug: 362138008

Change-Id: I78b8f210e379c79e3ea41afe4c4be3b24abf1fec
Signed-off-by: dongliang.cui <dongliang.cui@unisoc.com>