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qio: fix queue bloat blocking condition for qwrite()

The code in qwrite() that enters a non-interruptable
blocking loop is there because normally
flow control is done after Block's have been queued.

A process can skip that flow-control by receiving
a note duing qflow(). If that happens in a loop
it can bloat the queue without limit.

So if normal flow control fails to maintain the limit,
we want to block before allocating and queuing
the blocks as a last resort.

The condition however should be q->len >= q->limit,
not q->len/2 >= q->limit.

--- a/sys/src/9/port/qio.c

+++ b/sys/src/9/port/qio.c

@@ -1165,7 +1165,7 @@

 		print("qwrite hi %#p\n", getcallerpc(&q));

 	/* stop queue bloat before allocating blocks */

-	if(q->len/2 >= q->limit && q->noblock == 0 && q->bypass == nil){

+	if(q->len >= q->limit && q->noblock == 0 && q->bypass == nil){

 		while(waserror()){

 			if(up->procctl == Proc_exitme || up->procctl == Proc_exitbig)

 				error(Egreg);

@@ -1222,10 +1222,7 @@

 		ilock(q);

-		/* we use an artificially high limit for kernel prints since anything

-		 * over the limit gets dropped

-		 */

-		if((q->state & Qclosed) != 0 || q->len/2 >= q->limit){

+		if((q->state & Qclosed) != 0 || q->len >= q->limit){

 			iunlock(q);

 			freeb(b);

 			break;