md/md-bitmap: factor behind write counters out from bitmap_{start/end}write()

commit 08c50142 upstream.

behind_write is only used in raid1, prepare to refactor
bitmap_{start/end}write(), there are no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/r/20250109015145.158868-2-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>
[There is no bitmap_operations, resolve conflicts by exporting new api
md_bitmap_{start,end}_behind_write]
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>