runtime: simplify free count calculation in (un)cacheSpan

For unclear reasons, cacheSpan and uncacheSpan compute the number of
elements in a span by dividing its size by the element size. This
number is simply available in the mspan structure, so just use it.

Change-Id: If2e5de6ecec39befd3324bf1da4a275ad000932f
Reviewed-on: https://go-review.googlesource.com/c/138656
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mcentral.go

@@ -117,8 +117,7 @@ havespan:
 	if trace.enabled && !traceDone {
 		traceGCSweepDone()
 	}
-	cap := int32((s.npages << _PageShift) / s.elemsize)
-	n := cap - int32(s.allocCount)
+	n := int(s.nelems) - int(s.allocCount)
 	if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
 		throw("span has no free objects")
 	}
@@ -168,8 +167,7 @@ func (c *mcentral) uncacheSpan(s *mspan) {
 		atomic.Store(&s.sweepgen, sg)
 	}
 
-	cap := int32((s.npages << _PageShift) / s.elemsize)
-	n := cap - int32(s.allocCount)
+	n := int(s.nelems) - int(s.allocCount)
 	if n > 0 {
 		// cacheSpan updated alloc assuming all objects on s
 		// were going to be allocated. Adjust for any that