- fix possible prefetch-fault loop on NX page, based on suggestions
   from Jamie Lokier.

 - clean up nx feature dependencies

 - simplify detection of NX-violations when the kernel executes code

 - introduce pte_exec_kern() to simplify the NX logic

 - split the definitions out of pgtable-[23]level.h into
   pgtable-[23]level-defs.h, to enable the former to use generic
   pte functions from pgtable.h.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>