mm: allow pgtable_page_ctor() to fail

Change pgtable_page_ctor() return type from void to bool.  Returns true,
if initialization is successful and false otherwise.

Current implementation never fails, but it will change later.

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