Fix: check ht_del ret value of ust app session

UST app sesion can be destroyed by two execution paths. Either the app
unregisters or a destroy session is triggered. So, allowing a ht_del to
fail means that the session is already scheduled for teardown in a rcu
call.

Furthermore, this means that when looking up for a ust app session that
is not found becomes valid since it means it is in the teardown process.

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