block/copy-before-write: cbw_init(): use file child after attaching

block/copy-before-write: cbw_init(): use file child after attaching

In the next commit we'll get rid of source argument of cbw_init().
Prepare to it now, to make next commit simpler: move the code block
that uses source below attaching the child and use bs->file->bs instead
of source variable.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-17-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>