Fix: relayd: ressource leaks on viewer_stream_create error

Fix: relayd: ressource leaks on viewer_stream_create error

Observed issue
==============

When facing failure to open viewer stream chunks in the context of "Fix:
relayd: failure to open chunk files concurrently with session clear",
we observe that the relay daemon triggers an assertion due to a
non-empty session hash table on cleanup.

Cause
=====

viewer_stream_create() does a stream_get(), but without any matching
stream_put() on error. This in turn holds a reference on the ctf_trace,
which holds a reference on the session.

By inspecting the code, we notice that the following ressources can be
leaked on error:

- vstream->stream_file.handle,
- vstream->index_file,
- vstream->stream.

In non-error scenarios, viewer_stream_release() is responsible for
releasing references on the composite objects.

The vstream->stream_file.trace_chunk is not an issue because it is put
in the destroy handler (as well as within the release, before having its
vstream->stream_file.trace_chunk pointer set to NULL).

Solution
========

Properly put references on all objects which are contained by the viewer
stream on error by introducing
viewer_stream_release_composite_objects(), which is used both in the
error path of viewer_stream_create() and in viewer_stream_release().

Note
====

Why not move those "put" operations in viewer_stream_destroy ?

This is done in the release to ensure we put references on composite
objects immediately when our own reference reaches 0, rather than
waiting for a grace period through call_rcu, which could then cause
chained call_rcu callbacks and require multiple invocation of
rcu_barrier on relayd exit to guarantee that all callbacks have been
executed and all ressources properly freed.

Known drawbacks
===============

None.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I864b58eda94ebda5e6faea45c922d4e814a15daa