md/raid1: hold the barrier until handle_read_error() finishes

handle_read_error() will call allow_barrier() to match the former barrier
raising. However, it should put the allow_barrier() at the end to avoid a
concurrent raid reshape.

Fixes: 689389a0

 ("md/raid1: simplify handle_read_error().")
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Xueshi Hu <xueshi.hu@smartx.com>
Link: https://lore.kernel.org/r/20230814135356.1113639-4-xueshi.hu@smartx.com
Signed-off-by: Song Liu <song@kernel.org>