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

[ Upstream commit b2f11c6f3e1fc60742673b8675c95b78447f3dae ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

lib/generic-radix-tree.c

@@ -131,6 +131,8 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
 		if ((v = cmpxchg_release(&radix->root, r, new_root)) == r) {
 			v = new_root;
 			new_node = NULL;
+		} else {
+			new_node->children[0] = NULL;
 		}
 	}