Impact: fix debug_smp_processor_id() for CONFIG_CPUMASK_OFFSTACK=y

The scheduler now uses the new cpumask API, which deals up to
nr_cpumask_bits, whereas the API used NR_CPUS bits.

If CONFIG_CPUMASK_OFFSTACK=y these two are not equal, so the top bits
are undefined.  Leading to bug 12518 "BUG: using smp_processor_id() in
preemptible [00000000] code: dellWirelessCtl/..."

The fix is simple: use the modern API in the check.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>