powerpc/mm: move hugetlb_disabled into asm/hugetlb.h

No need to have this in asm/page.h, move it into asm/hugetlb.h

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>