mm/debug_pagealloc.c: don't allocate page_ext if we don't use guard page

mm/debug_pagealloc.c: don't allocate page_ext if we don't use guard page

What debug_pagealloc does is just mapping/unmapping page table.
Basically, it doesn't need additional memory space to memorize something.
But, with guard page feature, it requires additional memory to distinguish
if the page is for guard or not.  Guard page is only used when
debug_guardpage_minorder is non-zero so this patch removes additional
memory allocation (page_ext) if debug_guardpage_minorder is zero.

It saves memory if we just use debug_pagealloc and not guard page.

Link: http://lkml.kernel.org/r/1471315879-32294-3-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>