percpu_data blindly allocates bootmem memory to store NR_CPUS instances of 
cpudata, instead of allocating memory only for possible cpus.

This patch saves ram, allocating num_possible_cpus() (instead of NR_CPUS) 
instances.

This patch also makes sure a reference to  per_cpu(object, not_possible_cpu) 
does a reference to invalid memory (NULL+small_offset).

As some architectures (x86_64) are now allocating cpudata only on possible 
cpus, we (kernel developers on x86 machines) should make sure that x86 does a 
similar thing to find bugs. This is important that this patch has some 
exposure in -mm for some time, some places must now use :

for_each_cpu(i) {
	... per_cpu(xxx,i) ...

instead of the traditional

for (i = 0 ; i < NR_CPUS ; i++)


Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>