check_aio_block_status() was not using 3 of its parameters, and
block_status_filter() was open-coding the same error message.  The
choice of error message may change based on code ordering, but overall
there is no semantic change and less code maintenance.

Fixes: 8204e3ef ("api: Add [aio_]nbd_block_status_64")
Fixes: 0929d861

 ("api: Add nbd_[aio_]block_status_filter")
Signed-off-by: Eric Blake <eblake@redhat.com>