runtime: fix sanity check in notetsleep

CL 3660 replaced m.gcing with m.preemptoff, but unintentionally
reversed the sense of part of a sanity check in notetsleep.
Originally, notetsleep required that it be called from g0 or with
preemption disabled (specifically from within the garbage collector).
CL 3660 made it require that it be called from g0 or that preemption
be *enabled*.

I'm not sure why it had the original exception for being called from a
user g within the garbage collector, but the current garbage collector
certainly doesn't need that, and the new condition is completely wrong.

Make the sanity check just require that it's called on g0.

Change-Id: I6980d44f5a4461935e10b1b33a981e32b1b7b0c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/170063
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/lock_sema.go

@@ -262,7 +262,7 @@ func notetsleep_internal(n *note, ns int64, gp *g, deadline int64) bool {
 
 func notetsleep(n *note, ns int64) bool {
 	gp := getg()
-	if gp != gp.m.g0 && gp.m.preemptoff != "" {
+	if gp != gp.m.g0 {
 		throw("notetsleep not on g0")
 	}
 	semacreate(gp.m)