workqueue: only awaken a single sibling

There is no point in awakening two siblings that will fight to be the
first one to get a worker's workqueue. Just wake up one sibling at most.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
workqueue: preserve FIFO work order

Never merge work from a sibling into a non-empty worker queue, because
doing so does not keep FIFO ordering of the work.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>