Change USER/KERNEL_DS so that the merged version of
__strnlen_user can be used which allows us to complete the
removal of arch/ppc64/lib/.
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>

There is still a bug to be fixed and more merging to be done.
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>