dm raid: fix address sanitizer warning in raid_resume

There is a KASAN warning in raid_resume when running the lvm test
lvconvert-raid.sh. The reason for the warning is that mddev->raid_disks
is greater than rs->raid_disks, so the loop touches one entry beyond
the allocated length.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>