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: I4f6f2db39db990879ff10468c9db41606497bd6f