userfaultfd: selftest: avoid my_bcmp false positives with powerpc

Keep a non-zero placeholder after the count, for the my_bcmp comparison
of the page against the zeropage.  The lockless increment between 255 to
256 against a lockless my_bcmp could otherwise return false positives on
ppc32le.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>