Optimize directory access based on exfat_entry_set_cache.
 - Hold bh instead of copied d-entry.
 - Modify bh->data directly instead of the copied d-entry.
 - Write back the retained bh instead of rescanning the d-entry-set.
And
 - Remove unused cache related definitions.
Signed-off-by: NTetsuhiro Kohada <kohada.tetsuhiro@dc.mitsubishielectric.co.jp>
Reviewed-by: NSungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>