A patch from Martin Bligh which cleans up the open-coded uses of
mem_map for ia32.  Basically it replaces

mem_map + pagenr

with

pfn_to_page(pagenr)

in lots of places.  Because mem_map[] doesn't work like that with
discontigmem.

It also fixes a bug in bad_range, that happens to work for contig mem
systems, but is incorrect.  Tested both with and without discontigmem
support.