page flags: handle PG_uncached like all other flags

Remove the special setup for PG_uncached and simply make it part of the enum.
The page flag will only be allocated when the kernel build includes the
uncached allocator.

Acked-by: Dean Nelson <dcn@sgi.com>
Cc: Jes Sorensen <jes@trained-monkey.org>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>