Fix partition rollback issues on update failure.

BUG=chromium-os:22006
TEST=Ran command.

Change-Id: I19475945e3b2a16fe03805630a5e56fcbbfb1923
Reviewed-on: http://gerrit.chromium.org/gerrit/10585
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Dale Curtis <dalecurtis@chromium.org>
diff --git a/client/common_lib/cros/autoupdater.py b/client/common_lib/cros/autoupdater.py
index c183fe5..a5bb207 100644
--- a/client/common_lib/cros/autoupdater.py
+++ b/client/common_lib/cros/autoupdater.py
@@ -111,7 +111,7 @@
 
 
     def revert_boot_partition(self):
-        part = self.rootdev()
+        part = self.rootdev('-s')
         logging.warn('Reverting update; Boot partition will be %s', part)
         return self._run('/postinst %s 2>&1' % part)