vdso: don't scan MAP_GROWSDOWN vmas

* VDSO is never mapped with MAP_GROWSDOWN
* The first page  of growsdown vma may be a guard page, so any attempt
  to read it is suppressed by SIGBUS.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/x86/vdso.c

@@ -61,6 +61,9 @@ int parasite_fixup_vdso(struct parasite_ctl *ctl, pid_t pid,
 		if (vma->e->start > TASK_SIZE)
 			continue;
 
+		if (vma->e->flags & MAP_GROWSDOWN)
+			continue;
+
 		/*
 		 * I need to poke every potentially marked vma,
 		 * otherwise if task never called for vdso functions