'max_discard_sectors' is in sectors, while 'stripe' is in bytes.

This fixes the problem where DISCARD would get disabled on some larger
RAID5 configurations (6 or more drives in my testing), while it worked
as expected with smaller configurations.

Fixes: 620125f2 ("MD: raid5 trim support")
Cc: stable@vger.kernel.org v3.7+
Signed-off-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NShaohua Li <shli@fb.com>