Previously, radix_node's constructor would zero all of the pointers.
Now we use zalloc.  This fixes physical sharing between the mmap that
first allocates a radix node and every other operation that does
anything in that node (I argue this is not just sweeping this problem
under the rug: mmap obviously does not commute with address space
creation, which could theoretically pre-populate the radix tree.
Since that would consume too many resources, we do it lazily.  The
resulting physical sharing is exactly the same, it just happens at a
different time.)