Adds a gang lookup facility to radix trees.  It provides an efficient
means of locating a bunch of pages starting at a particular offset.

The implementation is a bit dumb, but is efficient enough.  And it is
amenable to the `tagged lookup' extension which is proving tricky to
write, but which will allow the dirty pages within a mapping to be
located in pgoff_t order.

Thanks are due to Huch Dickins for finding and fixing an unpleasant bug
in here.