block: bdrv_close: release bitmaps after drv->bdrv_close

Release bitmaps after 'if (bs->drv) { ... }' block. This will allow
format driver to save persistent bitmaps, which will appear in following
commits.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170628120530.31251-17-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>