be paranoid when marking workspace overflows

issue a membar to ensure that the overflow marker is written
before a failing WS allocation returns and is thus visible from
a different context.

pan_ic() already has an implicit membar via the mutex lock.

Ref varnishcache#2645

bin/varnishd/cache/cache_ws.c

@@ -31,6 +31,7 @@
 #include "config.h"
 
 #include "cache_varnishd.h"
+#include "vmb.h"
 
 #include <stdio.h>
 
@@ -115,6 +116,7 @@ WS_MarkOverflow(struct ws *ws)
 	CHECK_OBJ_NOTNULL(ws, WS_MAGIC);
 
 	ws->id[0] &= ~0x20;		// cheesy toupper()
+	VWMB();
 }
 
 static void