urcu-wait: Fix wait state load/store

urcu-wait: Fix wait state load/store

The state of a wait node must be accessed atomically. Also, the action
of busy loading until the teardown state is seen must follow a
CMM_ACQUIRE semantic while storing the teardown must follow a
CMM_RELEASE semantic.

Change-Id: I9cd9cf4cd9ab2081551d7f33c0b1c23c3cf3942f
Co-authored-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>