rcuhlist: make pointer stores atomic

Use rcu_assign_pointer() to store into head->next in
cds_hlist_add_head_rcu(). This includes the write barrier needed before
publishing the new node.

Use CMM_STORE_SHARED() to store into elem->prev->next in
cds_hlist_del_rcu().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>