x86: clean up using max_low_pfn on 32-bit

so that max_low_pfn is not changed after it is set.
so we can move that early and out of initmem_init.

could call find_low_pfn_range just after max_pfn is set.

also could move reserve_initrd out of setup_bootmem_allocator

so 32bit is more like 64bit.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>