wfcqueue: enqueue and splice return queue state

enqueue can return whether the queue was empty or not prior to enqueue.

splice can return this information about destination queue too, but
there are more cases to handle, because we don't touch the destination
queue if the source queue was empty, and in the nonblocking case, we
return that we would need to block on the source queue.

The destination queue state is sampled atomically with enqueue/splice to
destination operations.

Knowing this state is useful when "ownership" on a batch of queue items
can be assigned to those enqueuing the first items, e.g. to implement
wait/wakeup schemes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>