exfat: reuse exfat_find_location() to simplify exfat_get_dentry_set()

In exfat_get_dentry_set(), part of the code is the same as
exfat_find_location(), reuse exfat_find_location() to simplify
exfat_get_dentry_set().

Code refinement, no functional changes.

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>