SUNRPC: Convert the xprt->sending queue back to an ordinary wait queue

We no longer need priority semantics on the xprt->sending queue, because
the order in which tasks are sent is now dictated by their position in
the send queue.
Note that the backlog queue remains a priority queue, meaning that
slot resources are still managed in order of task priority.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>