rculist: Consolidate DEBUG_LIST for list_add_rcu()

This commit consolidates the debug checking for list_add_rcu() into the
new single __list_add_valid() debug function.  Notably, this commit fixes
the sanity check that was added in commit 17a801f4 ("list_debug:
WARN for adding something already in the list"), which wasn't checking
RCU-protected lists.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Rik van Riel <riel@redhat.com>