Fix: UST app session teardown process

This patch removes the ht_del of sessions from the delete_ust_app RCU
call and puts it in the unregister app function just before the call_rcu
is done.

To be able to free the sessions in the call rcu, a list is added for
which, when in tearing down an application or session, this list is used
to get the session reference for deletion.

Note that when in the RCU call, we are assured that the list is
exclusively accessed thus no need for any locking.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>