block/dirty-bitmaps: prohibit enable/disable on locked/frozen bitmaps

We're not being consistent about this. If it's in use by an operation,
the user should not be able to change the behavior of that bitmap.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20181002230218.13949-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>