runtime: steal the correct amount of GC assist credit

GC assists are supposed to steal at most the amount of background GC
credit available so that background GC credit doesn't go negative.
However, they are instead stealing the *total* amount of their debt
but only claiming up to the amount of credit that was available. This
results in draining the background GC credit pool too quickly, which
results in unnecessary assist work.

The fix is trivial: steal the amount of work we meant to steal (which
is already computed).

Change-Id: I837fe60ed515ba91c6baf363248069734a7895ef
Reviewed-on: https://go-review.googlesource.com/12643Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgcmark.go

@@ -182,7 +182,7 @@ func gcAssistAlloc(size uintptr, allowAssist bool) {
 		} else {
 			stolen = scanWork
 		}
-		xaddint64(&gcController.bgScanCredit, -scanWork)
+		xaddint64(&gcController.bgScanCredit, -stolen)
 
 		scanWork -= stolen
 		gp.gcscanwork += stolen