head.S creates the very initial pagetable for the kernel.  This just
maps enough space for the kernel itself, and an allocation bitmap.
The amount of mapped memory is rounded up to 4Mbytes, and so this
typically ends up mapping 8Mbytes of memory.

When booting, pagetable_init() needs to create mappings for all
lowmem, and the pagetables for these mappings are allocated from the
free pages around the kernel in low memory.  If the number of
pagetable pages + kernel size exceeds head.S's initial mapping, it
will end up faulting on an unmapped page.  This will only happen with
specific combinations of kernel size and memory size.

This patch makes sure that head.S also maps enough space to fit the
kernel pagetables as well as the kernel itself.  It ends up using an
additional two pages of unreclaimable memory.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Zachary Amsden <zach@vmware.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,