Impact: fix boot crash with numcpus=0 on certain systems

Fix early exception in __get_smp_config with nosmp.

Bail out early when there is no MP table.
Reported-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Tested-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>