dm verity: add error handling modes for corrupted blocks

Add device specific modes to dm-verity to specify how corrupted
blocks should be handled.  The following modes are defined:

  - 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.

In addition, each mode sends a uevent to notify userspace of
corruption and to allow further recovery actions.

The driver defaults to previous behavior (DM_VERITY_MODE_EIO)
and other modes can be enabled with an additional parameter to
the verity table.

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

Documentation/device-mapper/verity.txt

@@ -11,6 +11,7 @@ Construction Parameters
     <data_block_size> <hash_block_size>
     <num_data_blocks> <hash_start_block>
     <algorithm> <digest> <salt>
+    [<#opt_params> <opt_params>]
 
 <version>
     This is the type of the on-disk hash format.
@@ -62,6 +63,22 @@ Construction Parameters
 <salt>
     The hexadecimal encoding of the salt value.
 
+<#opt_params>
+    Number of optional parameters. If there are no optional parameters,
+    the optional paramaters section can be skipped or #opt_params can be zero.
+    Otherwise #opt_params is the number of following arguments.
+
+    Example of optional parameters section:
+        1 ignore_corruption
+
+ignore_corruption
+    Log corrupted blocks, but allow read operations to proceed normally.
+
+restart_on_corruption
+    Restart the system when a corrupted block is discovered. This option is
+    not compatible with ignore_corruption and requires user space support to
+    avoid restart loops.
+
 Theory of operation
 ===================