runtime: missed change from reorganization CL

That is, I accidentally dropped this change of Austin's
when preparing my CL. I blame Git.

Change-Id: I9dd772c84edefad96c4b16785fdd2dea04a4a0d6
Reviewed-on: https://go-review.googlesource.com/5320Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -423,7 +423,9 @@ func gcinstallmarkwb() {
 // This is the concurrent mark phase.
 //go:nowritebarrier
 func gcmark_m() {
-	gcDrain(nil)
+	var gcw gcWork
+	gcDrain(&gcw)
+	gcw.dispose()
 	// TODO add another harvestwbuf and reset work.nwait=0, work.ndone=0, and work.nproc=1
 	// and repeat the above gcDrain.
 }