reduce lock churn

Reference: /n/atom/patch/applied/clklkrdeuce
Date: Thu Jun 12 03:56:28 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/port/portclock.c	Thu Jun 12 03:56:14 2014
+++ /sys/src/nix/port/portclock.c	Thu Jun 12 03:56:15 2014
@@ -202,12 +202,12 @@
 		tt->head = t->tnext;
 		assert(t->tt == tt);
 		t->tt = nil;
-		iunlock(tt);
-		if(t->tf)
+		if(t->tf){
+			iunlock(tt);
 			(*t->tf)(u, t);
-		else
+			ilock(tt);
+		}else
 			callhzclock = 1;
-		ilock(tt);
 		if(t->tmode == Tperiodic)
 			tadd(tt, t);
 	}