MINOR: stream-int: don't needlessly call si_cs_send() in si_cs_process()

MINOR: stream-int: don't needlessly call si_cs_send() in si_cs_process()

There's a call there to si_cs_send() while we're supposed to come from
si_cs_io_cb() which has just done it. But in fact we can also come here
as a lower layer callback from ->wake() after a connection is established.
Since most of the time we'll end up here with either no data in the buffer
or a blocked output, let's simply check if we're already susbcribed to send
events before calling si_cs_send().