MINOR: stream-int: make it clear that si_ops cannot be null

MINOR: stream-int: make it clear that si_ops cannot be null

There was an ambiguity in which functions of the si_ops struct could be
null or not. only ->update doesn't exist in one of the si_ops (the
embedded one), all others are always defined. ->shutr and ->shutw were
never tested. However ->chk_rcv() and ->chk_snd() were tested, causing
confusion about the proper way to wake the other side up if undefined
(which never happens).

Let's update the comments to state these functions are mandatory and
remove the offending checks.