runtime: update outdated comments for deferprocStack

Change-Id: I0ea4d15da163cec6fe2a703376ce5a6032e15484
Reviewed-on: https://go-review.googlesource.com/c/go/+/714861
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>

Author: fengyoulin

src/runtime/panic.go

@@ -545,15 +545,13 @@ func deferprocStack(d *_defer) {
 	d.rangefunc = false
 	d.sp = sys.GetCallerSP()
 	// The lines below implement:
-	//   d.panic = nil
-	//   d.fd = nil
 	//   d.link = gp._defer
 	//   d.head = nil
 	//   gp._defer = d
-	// But without write barriers. The first three are writes to
+	// But without write barriers. The first two are writes to
 	// the stack so they don't need a write barrier, and furthermore
 	// are to uninitialized memory, so they must not use a write barrier.
-	// The fourth write does not require a write barrier because we
+	// The third write does not require a write barrier because we
 	// explicitly mark all the defer structures, so we don't need to
 	// keep track of pointers to them with a write barrier.
 	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))