MEDIUM: stream-int: call si_cs_process() in stream_int_update_conn

MEDIUM: stream-int: call si_cs_process() in stream_int_update_conn

Calling si_cs_send() alone is always dangerous because it can result
in the loss of an event if it manages to empty the buffer. Indeed, in
this case it's critical to call si_chk_rcv() on the opposite stream-int.
Given that si_cs_process() takes care of all this, let's call it instead.
All this code could possibly be refined soon to avoid redoing the whole
stream_int_notify() and do it only after a send(), but at the moment it's
not important.