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>