dm verity fec: fix block calculation

do_div was replaced with div64_u64 at some point, causing a bug with
block calculation due to incompatible semantics of the two functions.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Fixes: a739ff3f ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>