UPSTREAM: 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.

Change-Id: I728fa4b2586b29f2793ea5cb014289892819d249
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 0cc37c2df4fa0aa702f9662edce4b7ce12c86b7a)