Fix: missing rcu_read_lock in cmd_start_trace()

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: missing rcu_read_lock when calling trace_ust_find_agent()

Added a comment to note that this function is assumed to be
called with the rcu_read_lock held.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: missing rcu_read_lock in trace_ust_destroy_session()

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>