Instead of dereferencing node_start/end_pfn[] directly, make
init_alloc_remap() take @start and @end and let the caller be
responsible for making sure the range is sane.  This is to prepare for
use from unified NUMA init code.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>