Reduce the radix tree nodes from 128 slots to 64.

- The main reason for this is that on 64-bit/4k page machines, the
  slab allocator has decided that radix tree nodes will require an
  order-1 allocation.  Shrinking the nodes to 64 slots pulls that back
  to an order-0 allocation.

- On x86 we get fifteen 64-slot nodes per page rather than seven
  129-slot nodes, for a modest memory saving.

- Halving the node size will approximately halve the memory use in
  the worrisome really-large, really-sparse file case.

Of course, the downside is longer tree walks.  Each level of the tree
covers six bits of pagecache index rather than seven.  As ever, I am
guided by Anton's profiling on the 12- and 32-way PPC boxes.
radix_tree_lookup() is currently down in the noise floor.

Now, there is one special case: one file which is really big and which
is accessed in a random manner and which is accessed very heavily: the
blockdev mapping.  We _are_ showing some locking cost in
__find_get_block (used to be __get_hash_table) and in its call to
find_get_page().  I have a bunch of patches which introduce a generic
per-cpu buffer LRU, and which remove ext2's private bitmap buffer LRUs.
I expect these patches to wipe the blockdev mapping lookup lock contention
off the map,  but I'm awaiting test results from Anton before deciding
whether those patches are worth submitting.