The compound page logic is a little fragile - it relies on additional
metadata in the pageframes which some other kernel code likes to stomp on
(xfs was doing this).

Also, because we're treating all higher-order pages as compound pages it is
no longer possible to free individual lower-order pages from the middle of
higher-order pages.  At least one ARM driver insists on doing this.

We only really need the compound page logic for higher-order pages which can
be mapped into user pagetables and placed under direct-io.  This covers
hugetlb pages and, conceivably, soundcard DMA buffers which were allcoated
with a higher-order allocation but which weren't marked PageReserved.

The patch arranges for the hugetlb implications to allocate their pages with
compound page metadata, and all other higher-order allocations go back to the
old way.

(Andrea supplied the GFP_LEVEL_MASK fix)