dm verity: add ignore_zero_blocks feature

If ignore_zero_blocks is enabled dm-verity will return zeroes for blocks
matching a zero hash without validating the content.

Bug: 21893453
Change-Id: Ib9552f872bd82b1ba6a090686d2934a9551a3b48
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>