Zero out blocks that may be touched by dm-verity.

dm-verity may touch some blocks that are not in the care_map due to
block device read-ahead. It will fail if such blocks contain
non-zeroes. As a workaround, we mark them as extended blocks and
zero out explicitly to avoid dm-verity failures.

Bug: 20881595
Change-Id: I54e24e70ad822c0d6d7af43301f74d24505f4461