update_verifier should read dm wrapped partition

update_verifier used to read from system_block_device, which bypasses
dm-verity check completely. Switch update_verifier to read the corresponding
'/dev/block/dm-X' instead. U_v gets the verity block device number by
comparing the contents in '/sys/block/dm-X/dm/name'.

Bug: 34391662
Test: update_verifier detects the corrupped blocks and dm-verity trigger the reboot on Sailfish.
Change-Id: Ie5c50c23410bd29fcc6e733ba29cf892e9a07460