Divorces task xid<->request slot mapping from the internals of the
rpc_waitqueue xprt->pending. Instead xprt_lookup_rqst() is made to
search a dedicated list (xprt->recv) on which the request slot is
placed immediately after being allocated to a task. The new queue is
protected using the spinlock xprt->sock_lock rather than the generic
RPC task lock.

  Both udp_data_ready() and tcp_data_ready() (well tcp_read_request()
actually) now need to protect against the request being removed from
the xprt->recv list while they copy the RPC reply data from the skb.
On the other hand, they no longer need to worry about the task
disappearing from xprt->pending. This means that rpc_lock_task() hack
can be replaced by the spinlock xprt->sock_lock.