There was a problem that a user burned a dm-integrity image on CDROM
and could not activate it because it had a non-empty journal.

Fix this problem by flushing the journal (done by the previous commit)
and clearing the journal (done by this commit). Once the journal is
cleared, dm-integrity won't attempt to replay it on the next
activation.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>