rbd: check return values when scheduling aio

If scheduling fails, the number of outstanding I/Os must be correct,
or there will be a hang when waiting for everything to be flushed.

Reviewed-by: Christian Brunner <chb@muc.de>
Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
