asm-generic: introduce __ARCH_USE_5LEVEL_HACK

We are going to introduce <asm-generic/pgtable-nop4d.h> to provide
abstraction for properly (in opposite to 5level-fixup.h hack) folded
p4d level. The new header will be included from pgtable-nopud.h.

If an architecture uses <asm-generic/nop*d.h>, we cannot use
5level-fixup.h directly to quickly convert the architecture to 5-level
paging as it would conflict with pgtable-nop4d.h.

With this patch an architecture can define __ARCH_USE_5LEVEL_HACK before
inclusion <asm-genenric/nop*d.h> to use 5level-fixup.h.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>