SUNRPC: Avoid an unnecessary task reschedule on ENOTCONN

If the socket is unconnected, and xprt_transmit() returns ENOTCONN, we
currently give up the lock on the transport channel. Doing so means that
the lock automatically gets assigned to the next task in the xprt->sending
queue, and so that task needs to be woken up to do the actual connect.

The following patch aims to avoid that unnecessary task switch.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>