dm integrity: add recovery mode

In recovery mode, we don't:
- replay the journal
- check checksums
- allow writes to the device

This mode can be used as a last resort for data recovery.  The
motivation for recovery mode is that when there is a single error in the
journal, the user should not lose access to the whole device.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/device-mapper/dm-integrity.txt

@@ -59,6 +59,11 @@ Target arguments:
 		either both data and tag or none of them are written. The
 		journaled mode degrades write throughput twice because the
 		data have to be written twice.
+	R - recovery mode - in this mode, journal is not replayed,
+		checksums are not checked and writes to the device are not
+		allowed. This mode is useful for data recovery if the
+		device cannot be activated in any of the other standard
+		modes.
 
 5. the number of additional arguments