dm-verity: Add modes and emit uevent on corrupted blocks

Add a device specific mode to dm-verity for handling corrupted
blocks:

  DM_VERITY_MODE_EIO is the default behavior, where reading a
  corrupted block results in -EIO.

  DM_VERITY_MODE_LOGGING only logs corrupted blocks, but does
  not block the read.

  DM_VERITY_MODE_RESTART calls kernel_restart when a corrupted
  block is discovered.

Each mode sends a uevent to notify userspace of corruption and
allow further recovery actions.

Defaults to previous behavior, other modes can be enabled with
an optional parameter added to the verity table.

Change-Id: Ib72ae6ccb865594d28f3553bdcc5a40b1d7af390
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>