runtime: clarify access to mheap_.busy

There are two accesses to mheap_.busy that are guarded by checks
against len(mheap_.free). This works because both lists are (and must
be) the same length, but it makes the code less clear. Change these to
use len(mheap_.busy) so the access more clearly parallels the check.

Fixes #18944.

Change-Id: I9bacbd3663988df351ed4396ae9018bc71018311
Reviewed-on: https://go-review.googlesource.com/36354
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -589,7 +589,7 @@ func (h *mheap) alloc_m(npage uintptr, sizeclass int32, large bool) *mspan {
 			memstats.heap_objects++
 			atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift))
 			// Swept spans are at the end of lists.
-			if s.npages < uintptr(len(h.free)) {
+			if s.npages < uintptr(len(h.busy)) {
 				h.busy[s.npages].insertBack(s)
 			} else {
 				h.busylarge.insertBack(s)
@@ -941,7 +941,7 @@ func (h *mheap) freeList(npages uintptr) *mSpanList {
 }
 
 func (h *mheap) busyList(npages uintptr) *mSpanList {
-	if npages < uintptr(len(h.free)) {
+	if npages < uintptr(len(h.busy)) {
 		return &h.busy[npages]
 	}
 	return &h.busylarge