Make the following trivial changes in preparation for further updates.

* nodeid -> nid, nid -> tnid
* use nd_ prefix for nodedata related variables
* remove start/end_pfn and use start/end directly
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>