dm-verity: align struct dm_verity_fec_io properly

dm_verity_fec_io is placed after the end of two hash digests. If the hash
digest has unaligned length, struct dm_verity_fec_io could be unaligned.

This commit fixes the placement of struct dm_verity_fec_io, so that it's
aligned.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Fixes: a739ff3f

 ("dm verity: add support for forward error correction")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>