<asm/boot.h> needs <asm/pgtable_types.h>, not <asm/page_types.h> in
order to resolve PMD_SHIFT.  Also, correct a +1 which really should be
+ THREAD_ORDER.

This is a build error which was masked by a typoed #ifdef.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>