runtime: fix bug in preemption checks; was causing "lock count" panics

R=r
CC=golang-dev
https://golang.org/cl/186078

src/pkg/runtime/malloc.cgo

@@ -27,7 +27,7 @@ mallocgc(uintptr size, uint32 refflag, int32 dogc)
 	void *v;
 	uint32 *ref;
 
-	if(gcwaiting && g != m->g0)
+	if(gcwaiting && g != m->g0 && m->locks == 0)
 		gosched();
 	if(m->mallocing)
 		throw("malloc/free - deadlock");

src/pkg/runtime/proc.c

@@ -523,6 +523,8 @@ scheduler(void)
 void
 gosched(void)
 {
+	if(m->locks != 0)
+		throw("gosched holding locks");
 	if(g == m->g0)
 		throw("gosched of g0");
 	if(gosave(&g->sched) == 0)