md: raid1/raid10: initialize bvec table via bio_add_page()

We will support multipage bvec soon, so initialize bvec
table using the standardy way instead of writing the
talbe directly. Otherwise it won't work any more once
multipage bvec is enabled.

Acked-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>