radix-tree: move declarations to header

The xarray.c file contains the only call to radix_tree_node_rcu_free(),
and it comes with its own extern declaration for it.  This means the
function definition causes a missing-prototype warning:

lib/radix-tree.c:288:6: error: no previous prototype for 'radix_tree_node_rcu_free' [-Werror=missing-prototypes]

Instead, move the declaration for this function to a new header that can
be included by both, and do the same for the radix_tree_node_cachep
variable that has the same underlying problem but does not cause a warning
with gcc.

[zhangpeng.00@bytedance.com: fix building radix tree test suite]
  Link: https://lkml.kernel.org/r/20230521095450.21332-1-zhangpeng.00@bytedance.com
Link: https://lkml.kernel.org/r/20230516194212.548910-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>