md/md-bitmap: add 'events_cleared' into struct md_bitmap_stats

Also add a new helper to get events_cleared to avoid dereferencing
bitmap directly to prepare inventing a new bitmap.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-5-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>