Add rcu_read_ongoing() assertions around process_client_msg

process_client_msg ensures that RCU read-side lock should not be held
when calling it. Validate this using rcu_read_ongoing() at the entry and
exit points of this function. This allows us to catch unbalanced RCU
read-side lock within commands quickly.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>