md/md-bitmap: add a new method blocks_synced() in bitmap_operations

md/md-bitmap: add a new method blocks_synced() in bitmap_operations

Currently, raid456 must perform a whole array initial recovery to build
initail xor data, then IO to the array won't have to read all the blocks
in underlying disks.

This behavior will affect IO performance a lot, and nowadays there are
huge disks and the initial recovery can take a long time. Hence llbitmap
will support lazy initial recovery in following patches. This method is
used to check if data blocks is synced or not, if not then IO will still
have to read all blocks for raid456.

Link: https://lore.kernel.org/linux-raid/20250829080426.1441678-9-yukuai1@huaweicloud.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>