Write multiple sectors at once when updating dir-entries.
Add exfat_update_bhs() for that. It wait for write completion once
instead of sector by sector.
It's only effective if sync enabled.
Signed-off-by: NTetsuhiro Kohada <kohada.t2@gmail.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>