BACKPORT: dm verity: add 'check_at_most_once' option to only validate hashes once

BACKPORT: dm verity: add 'check_at_most_once' option to only validate hashes once

This allows platforms that are CPU/memory contrained to verify data
blocks only the first time they are read from the data device, rather
than every time.  As such, it provides a reduced level of security
because only offline tampering of the data device's content will be
detected, not online tampering.

Hash blocks are still verified each time they are read from the hash
device, since verification of hash blocks is less performance critical
than data blocks, and a hash block will not be verified any more after
all the data blocks it covers have been verified anyway.

This option introduces a bitset that is used to check if a block has
been validated before or not.  A block can be validated more than once
as there is no thread protection for the bitset.

These changes were developed and tested on entry-level Android Go
devices.

Change-Id: I4a0be2552acb04f7c2ce6665031199d9dcbc2430
Bug: 72664474
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 843f38d382b1ca2f6f4ae2ef7c35933e6319ffbb)
(changed alloc to vzalloc/vfree)
Signed-off-by: Patrik Torstensson <totte@google.com>