num_possible_cpus() can be > 1 when disabled CPUs have been accounted.

Disabled CPUs are not in the cpu_present_map, so we can use
num_present_cpus() as a safe indicator to switch to UP alternatives.
Reported-by: NChuck Ebbert <cebbert@redhat.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: <stable@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>