UPSTREAM: dm verity: add ignore_zero_blocks feature

(cherry-picked from 0cc37c2df4fa0aa702f9662edce4b7ce12c86b7a)

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: Ief4cc7c07d6166fa33fab5faf7de8b9a4a470656
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>