smp: Avoid 'setup_max_cpus' namespace collision/shadowing

bringup_nonboot_cpus() gets passed the 'setup_max_cpus'
variable in init/main.c - which is also the name of the parameter,
shadowing the name.

To reduce confusion and to allow the 'setup_max_cpus' value
to be #defined in the <linux/smp.h> header, use the 'max_cpus'
name for the function parameter name.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org