arch/powerpc/platforms/pseries/hotplug-memory.c: section removal cleanups

Followups to d760afd4 ("memory-hotplug: suppress "Trying to free
nonexistent resource <XXXXXXXXXXXXXXXX-YYYYYYYYYYYYYYYY>" warning").

 - use unsigned long type, as overflows are conceivable

 - rename `i' to the less-misleading and more informative `section'

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>