rcu: List-debug variants of rcu list routines.

* Make __list_add_rcu check the next->prev and prev->next pointers
  just like __list_add does.
* Make list_del_rcu use __list_del_entry, which does the same checking
  at deletion time.

Has been running for a week here without anything being tripped up,
but it seems worth adding for completeness just in case something
ever does corrupt those lists.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>