block: add missing coroutine_fn to bdrv_sum_allocated_file_size()

Not a coroutine_fn, you say?

  static int64_t bdrv_sum_allocated_file_size(BlockDriverState *bs)
  {
      BdrvChild *child;
      int64_t child_size, sum = 0;

      QLIST_FOREACH(child, &bs->children, next) {
          if (child->role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
                             BDRV_CHILD_FILTERED))
          {
              child_size = bdrv_co_get_allocated_file_size(child->bs);
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Well what do we have here?!

I rest my case, your honor.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230308211435.346375-1-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>