x86: add table_top check for alloc_low_page in 64 bit

that range is from find_e820_area, so don't try to use end_pfn to see
if out of boundary...use table_top instead to avoid possible strange
result while cross the boundary...

also change early_printk to printk, because init_memory_mapping is after
early param parsing, and console=uart8250 already working at that time.

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