x86: prepare page allocator for high allocations on PAGEALLOC=y

To preserve the DMA pool in CONFIG_DEBUG_PAGEALLOC=y kernels, we'll
allocate pagetables from above the 16MB DMA limit, so we'll have to set
up boot pagetables to cover 16MB more RAM (worst-case).

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>