From Martin Bligh

This patch fixes a nasty bug that wli found in buffer.c which
cause an oops - we were using contig_page_data on a discontigmem
machine. It's a slightly modified version of the fix wli suggested,
tested on NUMA-Q.

I've also added code to not define contig_page_data for discontigmem
systems, to stop this from happening again. I wrapped a couple of
bootmem functions that were using it in #ifndef CONFIG_DISCONTIGMEM.
I suppose it's possible (though unlikely) that some other discontig
arch might need to wrap a couple of functions in their tree similarly,
but any borkage will just give a simple clear compiler error telling
them exactly where the problem is.