mm/page_cgroup.c: simplify code by using SECTION_ALIGN_UP() and SECTION_ALIGN_DOWN() macros

Commit a539f353 ("mm: add SECTION_ALIGN_UP() and
SECTION_ALIGN_DOWN() macro") introduced the SECTION_ALIGN_UP() and
SECTION_ALIGN_DOWN() macros.  Use those macros to increase code
readability.

Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>