This adds a hack to /dev/mem (along with the other ones already there)
to prevent mapping cacheable of the IO hole.

Without this, XFree blows up on machines with enough memory to go past
the IO hole.  It also tries to prevent memory from beeing mapped
uncached.  Cache paradoxes are evil and can kill the CPU.

The necessary page_is_ram() call was added by the previous patch doing
the proper IO hole accounting.