openrisc: add missing pgtable_page_ctor/dtor calls

It will fix NR_PAGETABLE accounting.  It's also required if the arch is
going ever support split ptl.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>