runtime: make notetsleep_internal nowritebarrier

When notetsleep_internal is called from notetsleepg, notetsleepg has
just given up the P, so write barriers are not allowed in
notetsleep_internal.

Change-Id: I1b214fa388b1ea05b8ce2dcfe1c0074c0a3c8870
Reviewed-on: https://go-review.googlesource.com/12647Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/lock_futex.go

@@ -144,7 +144,11 @@ func notesleep(n *note) {
 	}
 }
 
+// May run with m.p==nil if called from notetsleep, so write barriers
+// are not allowed.
+//
 //go:nosplit
+//go:nowritebarrier
 func notetsleep_internal(n *note, ns int64) bool {
 	gp := getg()