runtime: correct stats in SysFree

R=r
CC=golang-dev
https://golang.org/cl/2254047

src/pkg/runtime/darwin/mem.c

@@ -21,6 +21,7 @@ SysUnused(void *v, uintptr n)
 void
 SysFree(void *v, uintptr n)
 {
+	mstats.sys -= n;
 	runtime_munmap(v, n);
 }
 

src/pkg/runtime/freebsd/mem.c

@@ -21,6 +21,7 @@ SysUnused(void *v, uintptr n)
 void
 SysFree(void *v, uintptr n)
 {
+	mstats.sys -= n;
 	runtime_munmap(v, n);
 }
 

src/pkg/runtime/linux/mem.c

@@ -33,6 +33,7 @@ SysUnused(void *v, uintptr n)
 void
 SysFree(void *v, uintptr n)
 {
+	mstats.sys -= n;
 	runtime_munmap(v, n);
 }
 

src/pkg/runtime/nacl/mem.c

@@ -26,6 +26,7 @@ void
 SysFree(void *v, uintptr n)
 {
 	// round to page size or else nacl prints annoying log messages
+	mstats.sys -= n;
 	n = (n+NaclPage-1) & ~(NaclPage-1);
 	runtime_munmap(v, n);
 }