Fix: Missing rcu_read_lock in ust_app_get_nb_stream()

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

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

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