Patch from Ravikiran G Thirumalai <kiran@in.ibm.com>

1. Break out disk stats from kernel_stat and move disk stat to blkdev.h

2. Group cpu stat in kernel_stat and make them "per_cpu" instead of
   the NR_CPUS array

3. Remove EXPORT_SYMBOL(kstat) from ksyms.c (as I noticed that no module is
   using kstat)