UPSTREAM: exfat: call bh_read in get_block only when necessary

With commit 11a347fb ("exfat: change to get file size from DataLength"),
exfat_get_block() can now handle valid_size. However, most partial
unwritten blocks that could be mapped with other blocks are being
inefficiently processed separately as individual blocks.

Except for partial unwritten blocks that require independent processing,
let's handle them simply as before.

Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

(cherry-picked from commit c73e680d)
Change-Id: I094eb70166ed48dcfdea9a3fd234bf4b71289e33
Bug: 412215407
Signed-off-by: Paul Lawrence <paullawrence@google.com>