The patch implements the atomic copy_*_user() function.

If the kernel takes a pagefault while running copy_*_user() in an
atomic region, the copy_*_user() will fail (return a short value).

And with this patch, holding an atomic kmap() puts the CPU into an
atomic region.

- Increment preempt_count() in kmap_atomic() regardless of the
  setting of CONFIG_PREEMPT.  The pagefault handler recognises this as
  an atomic region and refuses to service the fault.  copy_*_user will
  return a non-zero value.

- Attempts to propagate the in_atomic() predicate to all the other
  highmem-capable architectures' pagefault handlers.  But the code is
  only tested on x86.

- Fixed a PPC bug in kunmap_atomic(): it forgot to reenable
  preemption if HIGHMEM_DEBUG is turned on.

- Fixed a sparc bug in kunmap_atomic(): it forgot to reenable
  preemption all the time, for non-fixmap pages.

- Fix an error in <linux/highmem.h> - in the CONFIG_HIGHMEM=n case,
  kunmap_atomic() takes an address, not a page *.