dm verity: Fix IO priority lost when reading FEC and hash

After obtaining the data, verification or error correction process may
trigger a new IO that loses the priority of the original IO, that is,
the verification of the higher priority IO may be blocked by the lower
priority IO.

Make the IO used for verification and error correction follow the
priority of the original IO.

Co-developed-by: Yibin Ding <yibin.ding@unisoc.com>
Signed-off-by: Yibin Ding <yibin.ding@unisoc.com>
Signed-off-by: Hongyu Jin <hongyu.jin@unisoc.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>