page-types: constify read only arrays

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>