Sort-of-but-not-really from High Dickins.

We're doing a lot of buslocked operations in the page allocator just
for debug.  Plus when they _do_ trigger, there are so many BUG_ONs in
there that it's rather hard to work out from user reports which one
actually triggered.

So redo all that and also print out some more useful info about the
page state before taking the machine out.

(And yes, we need to take the machine out.  Incorrect page handling in
there can cause file corruption).