syscall: rename z_object_recycle -> k_object_recycle

Rename z_object_recycle and do not use z_ for internal APIs.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/userspace.c b/kernel/userspace.c
index 6a4a65a..3147275 100644
--- a/kernel/userspace.c
+++ b/kernel/userspace.c
@@ -779,7 +779,7 @@
 	ko->flags |= K_OBJ_FLAG_INITIALIZED;
 }
 
-void z_object_recycle(const void *obj)
+void k_object_recycle(const void *obj)
 {
 	struct k_object *ko = z_object_find(obj);