smp: Provide 'setup_max_cpus' definition on UP too

commit 3c2f8859 upstream.

This was already defined locally by init/main.c, but let's make
it generic, as arch/x86/kernel/cpu/topology.c is going to make
use of it to have more uniform code.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>