runtime: exit getfull barrier if there are partial workbufs

Currently, we only exit the getfull barrier if there is work on the
full list, even though the exit path will take work from either the
full or partial list. Change this to exit the barrier if there is work
on either the full or partial lists.

I believe it's currently safe to check only the full list, since
during mark termination there is no reason to put a workbuf on a
partial list. However, checking both is more robust.

Change-Id: Icf095b0945c7cad326a87ff2f1dc49b7699df373
Reviewed-on: https://go-review.googlesource.com/7840Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcwork.go

@@ -419,7 +419,7 @@ func getfull(entry int) *workbuf {
 
 	xadd(&work.nwait, +1)
 	for i := 0; ; i++ {
-		if work.full != 0 {
+		if work.full != 0 || work.partial != 0 {
 			xadd(&work.nwait, -1)
 			b = (*workbuf)(lfstackpop(&work.full))
 			if b == nil {