lib/generic-radix-tree.c: Fix rare race in __genradix_ptr_alloc()

If we need to increase the tree depth, allocate a new node, and then
race with another thread that increased the tree depth before us, we'll
still have a preallocated node that might be used later.

If we then use that node for a new non-root node, it'll still have a
pointer to the old root instead of being zeroed - fix this by zeroing it
in the cmpxchg failure path.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>