block: keep bitmap if incremental backup job is cancelled

Reclaim the dirty bitmap if an incremental backup block job is
cancelled.  The ret variable may be 0 when the job is cancelled so it's
not enough to check ret < 0.

Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1434380534-7680-1-git-send-email-stefanha@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
