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

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 5f7bfeb6466ffd17e874b84a4fcc826bb51714e7
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.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>