Fix __NR_map_shadow_stack syscall-renumbering fallout in the
x86 self-test code.

[ Arguably the existing code was unnecessarily fragile, and
  tooling should have picked up the new syscall number,
  and a wider fix is being worked on - but meanwhile, let's
  not have the old syscall number in the kernel tree. ]

Signed-off-by: Ingo Molnar <mingo@kernel.org>