runtime: fix nmspinning comparison

nmspinning has a value range of [0, 2^31-1].  Update the comment to
indicate this and fix the comparison so it's not always false.

Fixes #11280

Change-Id: Iedaf0654dcba5e2c800645f26b26a1a781ea1991
Reviewed-on: https://go-review.googlesource.com/13877Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/proc1.go

@@ -1538,7 +1538,7 @@ func resetspinning() {
 	if _g_.m.spinning {
 		_g_.m.spinning = false
 		nmspinning = xadd(&sched.nmspinning, -1)
-		if nmspinning < 0 {
+		if int32(nmspinning) < 0 {
 			throw("findrunnable: negative nmspinning")
 		}
 	} else {

src/runtime/runtime2.go

@@ -429,7 +429,7 @@ type schedt struct {
 
 	pidle      puintptr // idle p's
 	npidle     uint32
-	nmspinning uint32
+	nmspinning uint32 // limited to [0, 2^31-1]
 
 	// Global runnable queue.
 	runqhead guintptr