runtime: fix potential GC deadlock

The issue seems to not be triggered right now,
but I've seen the deadlock after some other legal
modifications to runtime.
So I think we are safer this way.

R=rsc, r
CC=golang-dev
https://golang.org/cl/6339051

src/pkg/runtime/mgc0.c

@@ -905,12 +905,14 @@ runtime·gc(int32 force)
 	work.debugmarkdone = 0;
 	work.nproc = runtime·gcprocs();
 	addroots();
+	m->locks++;	// disable gc during mallocs in parforalloc
 	if(work.markfor == nil)
 		work.markfor = runtime·parforalloc(MaxGcproc);
 	runtime·parforsetup(work.markfor, work.nproc, work.nroot, nil, false, markroot);
 	if(work.sweepfor == nil)
 		work.sweepfor = runtime·parforalloc(MaxGcproc);
 	runtime·parforsetup(work.sweepfor, work.nproc, runtime·mheap.nspan, nil, true, sweepspan);
+	m->locks--;
 	if(work.nproc > 1) {
 		runtime·noteclear(&work.alldone);
 		runtime·helpgc(work.nproc);