md: factor out helper to start reshape from action_store()

There are no functional changes, just to make code cleaner and prepare
for following refactor.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240611132251.1967786-5-yukuai1@huaweicloud.com