mm fix commmets: if SPARSEMEM, pgdata doesn't have page_ext

If SPARSEMEM, use page_ext in mem_section
if !SPARSEMEM, use page_ext in pgdata

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>