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
reading a couple of pages past the end of the vdso text.  This should send
SIGBUS instead of OOPSing.

The bug was introduced by:

commit 7a59ed415f5b57469e22e41fc4188d5399e0b194
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.

This will be fixed separately in 3.15, but that patch will not apply to
tip/x86/vdso.  This is the equivalent fix for tip/x86/vdso and,
presumably, 3.16.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stefani Seibold <stefani@seibold.net>
Cc: <stable@vger.kernel.org>	[needs rework for 3.15 and earlier]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>