net: lwm2m: Fix data cache rollback logic compilation

The root cause of this issue is a modification of `struct ring_buf` in
3075a7d9. Even though all the fields of the struct are marked as
internal, the LwM2M code is using some of them to roll back the state of
the ring buffer on failure.

Signed-off-by: Etienne de Maricourt <edmecomemail@gmail.com>