Fix a radix-tree bug spotted by Vladimir Saveliev <vs@namesys.com>.

Each step in the radix tree spans six address bits.  So a height=6 tree
spans 36-bits worth of nodes.

On 32-bit machines radix_tree_gang_lookup() doesn't handle this right -
at the 12TB mark it wraps back to zero, and returns pages at quite
wrong indices.

The patch fixes all that up, and tidies a couple of things.

A user-space test harness was developed so that the code can be sanely
tested.  It is at

	http://www.zip.com.au/~akpm/linux/patches/stuff/rtth.tar.gz