People love to do comparisons with highmem_start_page.  However, where
CONFIG_HIGHMEM=y and there is no actual highmem, there's no real page at
*highmem_start_page.

That's usually not a problem, but CONFIG_NONLINEAR is a bit more strict and
catches the bogus address tranlations. 

There are about a gillion different ways to find out of a 'struct page' is
highmem or not.  Why not just check page_flags?  Just use PageHighMem()
wherever there used to be a highmem_start_page comparison.  Then, kill off
highmem_start_page.

This removes more code than it adds, and gets rid of some nasty
#ifdefs in .c files.
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>