md/raid1-10: factor out a helper to submit normal write

md/raid1-10: factor out a helper to submit normal write

[ Upstream commit 8295efbe68c080047e98d9c0eb5cb933b238a8cb ]

There are multiple places to do the same thing, factor out a helper to
prevent redundant code, and the helper will be used in following patch
as well.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230529131106.2123367-4-yukuai1@huaweicloud.com
Stable-dep-of: 7db922bae3ab ("md/raid1-10: submit write io directly if bitmap is not enabled")
Signed-off-by: Sasha Levin <sashal@kernel.org>