rculist.h: use the rcu API

Make almost all list mutation primitives use rcu_assign_pointer().

The main point of this being readability improvement.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Josh Triplett <josh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>