dm-crypt: use bi_sector in bio when initialize integrity seed

bio->bi_iter.bi_sector has already been initialized when initialize the
integrity seed in dm_crypt_integrity_io_alloc(). There is no need to
calculate it again. Therefore, use the helper bip_set_seed() to
initialize the seed and pass bi_iter.bi_sector to it instead.

Mikulas: We can't use bip_set_seed because it doesn't compile without
CONFIG_BLK_DEV_INTEGRITY.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>