dm-integrity: set discard_granularity to logical block size

dm-integrity could set discard_granularity lower than the logical block
size. This could result in failures when sending discard requests to
dm-integrity.

This fix is needed for kernels prior to 6.10.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: Eric Wheeler <linux-integrity@lists.ewheeler.net>
Cc: stable@vger.kernel.org # <= 6.9
Signed-off-by: Mike Snitzer <snitzer@kernel.org>