x86: fix pte allocation in "x86: introduce init_memory_mapping for 32bit"

The patch "x86: introduce init_memory_mapping for 32bit" does not allocate
enough space for PTEs if the CPU does not implement PSE.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>