Presently the root node is initialized by way of kzalloc on the parent
data structure, which by chance happens to do the bulk of what an
explicit initialization does with GFP_NOWAIT semantics. This however is
more by luck than by design, and as we ideally want to permit radix node
allocations access to the emergency pools anyways, add in the proper
initializer with the desired mask.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>