Avoid false positive warnings in kmap_atomic_prot() with DEBUG_HIGHMEM

I believe http://bugzilla.kernel.org/show_bug.cgi?id=10318 is a false
positive.  There's no way in which networking will be using highmem pages
here, so it won't be taking the KM_USER0 kmap slot, so there's no point in
performing these checks.

Cc: Pawel Staszewski <pstaszewski@artcom.pl>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Christoph Lameter <clameter@sgi.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
 [ Really sad.  We lose almost all real-life coverage of the debug tests
   with this patch. Now it will only report problems for the cases where
   people actually end up using a HIGHMEM page, not when they just _might_
   use one.    - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>