runtime: unset m->locks after actual lock unlock

This is needed for preemptive scheduler,
it will preempt only when m->locks==0,
and we do not want to be preempted while
we have not completely unlocked the lock.

R=golang-dev, khr, iant
CC=golang-dev
https://golang.org/cl/9196047

src/pkg/runtime/lock_futex.c

@@ -91,14 +91,14 @@ runtime·unlock(Lock *l)
 {
 	uint32 v;
 
-	if(--m->locks < 0)
-		runtime·throw("runtime·unlock: lock count");
-
 	v = runtime·xchg((uint32*)&l->key, MUTEX_UNLOCKED);
 	if(v == MUTEX_UNLOCKED)
 		runtime·throw("unlock of unlocked lock");
 	if(v == MUTEX_SLEEPING)
 		runtime·futexwakeup((uint32*)&l->key, 1);
+
+	if(--m->locks < 0)
+		runtime·throw("runtime·unlock: lock count");
 }
 
 // One-time notifications.

src/pkg/runtime/lock_sema.c

@@ -93,9 +93,6 @@ runtime·unlock(Lock *l)
 	uintptr v;
 	M *mp;
 
-	if(--m->locks < 0)
-		runtime·throw("runtime·unlock: lock count");
-
 	for(;;) {
 		v = (uintptr)runtime·atomicloadp((void**)&l->key);
 		if(v == LOCKED) {
@@ -112,6 +109,9 @@ runtime·unlock(Lock *l)
 			}
 		}
 	}
+
+	if(--m->locks < 0)
+		runtime·throw("runtime·unlock: lock count");
 }
 
 // One-time notifications.