x86, mm: s/PAGES_PER_ELEMENT/PAGES_PER_SECTION/

DISCONTIGMEM on x86-32 implements pfn -> nid mapping similarly to
SPARSEMEM; however, it calls each mapping unit ELEMENT instead of
SECTION.  This patch renames it to SECTION so that PAGES_PER_SECTION
is valid for both DISCONTIGMEM and SPARSEMEM.  This will be used by
the next patch to implement mapping granularity check.

This patch is trivial constant rename.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20110712074422.GA2872@htj.dyndns.org


Cc: Hans Rosenfeld <hans.rosenfeld@amd.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>