Zero blocks before BLKDISCARD

Due to observed BLKDISCARD flakiness, overwrite blocks that we want
to discard with zeros first to avoid later issues with dm-verity if
BLKDISCARD is not successful.

Bug: 20614277
Bug: 20881595
Change-Id: I0280fe115b020dcab35f49041fb55b7f8e793da3