x86: don't reallocate pgt for node0

kva ram already mapped right after away, so don't need to get that for low ram.
avoid wasting one copy of pgdat.

also add node id in early_res name in case we get it from find_e820_area.

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