sd: Fix discard granularity when LBPRZ=1

Commit 39773722 ("sd: Make discard granularity match logical block
size when LBPRZ=1") accidentally set the granularity to one byte instead
of one logical block on devices that provide deterministic zeroes after
UNMAP.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reported-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ewan Milne <emilne@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Fixes: 39773722
Cc: <stable@vger.kernel.org> #v4.4+