exfat: rename exfat_free_dentry_set() to exfat_put_dentry_set()

Since struct exfat_entry_set_cache is allocated from stack,
no need to free, so rename exfat_free_dentry_set() to
exfat_put_dentry_set(). After renaming, the new function pair
is exfat_get_dentry_set()/exfat_put_dentry_set().

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>