powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff

powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff

Swapoff after swapping hangs on the G5, when CONFIG_CHECKPOINT_RESTORE=y
but CONFIG_MEM_SOFT_DIRTY is not set.  That's because the non-zero
_PAGE_SWP_SOFT_DIRTY bit, added by CONFIG_HAVE_ARCH_SOFT_DIRTY=y, is not
discounted when CONFIG_MEM_SOFT_DIRTY is not set: so swap ptes cannot be
recognized.

(I suspect that the peculiar dependence of HAVE_ARCH_SOFT_DIRTY on
CHECKPOINT_RESTORE in arch/powerpc/Kconfig comes from an incomplete
attempt to solve this problem.)

It's true that the relationship between CONFIG_HAVE_ARCH_SOFT_DIRTY and
and CONFIG_MEM_SOFT_DIRTY is too confusing, and it's true that swapoff
should be made more robust; but nevertheless, fix up the powerpc ifdefs
as x86_64 and s390 (which met the same problem) have them, defining the
bits as 0 if CONFIG_MEM_SOFT_DIRTY is not set.

Fixes: 7207f43665b8 ("powerpc/mm: Add page soft dirty tracking")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>