x86/cpu: Improve readability of per-CPU cpumask initialization code

In smp_prepare_cpus_common() and x2apic_prepare_cpu():

 - use 'cpu' instead of 'i'
 - use 'node' instead of 'n'
 - use vertical alignment to improve readability
 - better structure basic blocks
 - reduce col80 checkpatch damage

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org