md/raid1: free the r1bio before waiting for blocked rdev

Raid1 reshape will change mempool and r1conf::raid_disks which are
needed to free r1bio. allow_barrier() make a concurrent raid1_reshape()
possible. So, free the in-flight r1bio before waiting blocked rdev.

Fixes: 6bfe0b49

 ("md: support blocking writes to an array on device failure")
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Xueshi Hu <xueshi.hu@smartx.com>
Link: https://lore.kernel.org/r/20230814135356.1113639-3-xueshi.hu@smartx.com
Signed-off-by: Song Liu <song@kernel.org>