verity: also check corruption in existing mounts

If we are mounting a mol where one of the atoms already been mounted,
then we should check the already-mounted devices for any corruption
reported by devicemapper as well, and at least fail to mount the new
image using them.

Signed-off-by: Michael McCracken <[email protected]>

molecule.go

@@ -61,6 +61,10 @@ func (m Molecule) mountUnderlyingAtoms() error {
 				if err != nil {
 					return err
 				}
+				err = squashfs.ConfirmExistingVerityDeviceCurrentValidity(mountpoint.Source)
+				if err != nil {
+					return err
+				}
 			}
 			continue
 		}