From: Christoph Hellwig <hch@lst.de>

- don't include mempolicy.h in sched.h and mm.h when a forward delcaration
  is enough.  Andi argued against that in the past, but I'd really hate to add
  another header to two of the includes used in basically every driver when we
  can include it in the six files actually needing it instead (that number is
  for my ppc32 system, maybe other arches need more include in their
  directories)

- make numa api fields in tast_struct conditional on CONFIG_NUMA, this gives
  us a few ugly ifdefs but avoids wasting memory on non-NUMA systems.