x86, vdso: Fix an OOPS accessing the HPET mapping w/o an HPET

The oops can be triggered in qemu using -no-hpet (but not nohpet) by
running a 32-bit program and reading a couple of pages before the vdso.
This should send SIGBUS instead of OOPSing.

The bug was introduced by:

commit 7a59ed41
Author: Stefani Seibold <stefani@seibold.net>
Date:   Mon Mar 17 23:22:09 2014 +0100

    x86, vdso: Add 32 bit VDSO time support for 32 bit kernel

which is new in 3.15.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/e99025d887d6670b6c4d81e6ccfeeb83770b21e9.1400109621.git.luto@amacapital.netSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/vdso32-setup.c

@@ -155,6 +155,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	unsigned long addr;
 	int ret = 0;
 	struct vm_area_struct *vma;
+	static struct page *no_pages[] = {NULL};
 
 #ifdef CONFIG_X86_X32_ABI
 	if (test_thread_flag(TIF_X32))
@@ -193,7 +194,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 			addr -  VDSO_OFFSET(VDSO_PREV_PAGES),
 			VDSO_OFFSET(VDSO_PREV_PAGES),
 			VM_READ,
-			NULL);
+			no_pages);
 
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);