[PATCH] highmem: catch illegal nesting

Catch illegally nested kmap_atomic()s even if the page that is mapped by
the 'inner' instance is from lowmem.

This avoids spuriously zapped kmap-atomic ptes and turns hard to find
crashes into clear asserts at the bug site.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>