testing/radix-tree/maple: Hack around kfree_rcu not existing

testing/radix-tree/maple: Hack around kfree_rcu not existing

liburcu doesn't have kfree_rcu (or anything similar). Despite that, we
can hack around it in a trivial fashion, by adding a wrapper.

The wrapper only works for maple_nodes because we cannot get the
kmem_cache pointer any other way in the test code.

Link: https://lore.kernel.org/all/20250812162124.59417-1-pfalcato@suse.de/
Suggested-by: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>