runtime: consistency check for G rescan position

Issue #17099 shows a failure that indicates we rescanned a stack twice
concurrently during mark termination, which suggests that the rescan
list became inconsistent. Add a simple check when we dequeue something
from the rescan list that it claims to be at the index where we found
it.

Change-Id: I6a267da4154a2e7b7d430cb4056e6bae978eaf62
Reviewed-on: https://go-review.googlesource.com/29280
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

Author: aclements

src/runtime/mgcmark.go

@@ -199,6 +199,11 @@ func markroot(gcw *gcWork, i uint32) {
 			gp = allgs[i-baseStacks]
 		} else if baseRescan <= i && i < end {
 			gp = work.rescan.list[i-baseRescan].ptr()
+			if gp.gcRescan != int32(i-baseRescan) {
+				// Looking for issue #17099.
+				println("runtime: gp", gp, "found at rescan index", i-baseRescan, "but should be at", gp.gcRescan)
+				throw("bad g rescan index")
+			}
 		} else {
 			throw("markroot: bad index")
 		}