md/md-bitmap: cleanup bitmap_ops->startwrite()

bitmap_startwrite() always return 0, and the caller doesn't check return
value as well, hence change the method to void.

Also rename startwrite/endwrite to start_write/end_write, which is more in
line with the usual naming convention.

Link: https://lore.kernel.org/linux-raid/20250524061320.370630-4-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>