Fix: Missing rcu_read_lock in get_session_max_subbuf_size()

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: UST subbuffers silently dropped on moderate trace traffic

Well, it looks like we really screwed up on this one.

lttng-tools commit 02b3d1769d5f8a33e4109b1e681141c9295dfda6 introduced
an important regression for lttng-ust tracing in the consumer daemon:
after reading a sub-buffer, a check has been added to see whether there
are more sub-buffers available to read, and if it is the case, it
ensures the wakeup pipe will be awakened again.

The issue lies in the use of ustctl_put_next_subbuf() in this check.
This acts as if the sub-buffer has been read, when in reality it has not
been read. It therefore trashes the data contained by this sub-buffer.

This check should use ustctl_put_subbuf(), which does not move the
consumer position.

This is a severe bug, and the fix needs to be applied to stable-2.6,
stable-2.5, and stable-2.4.

Fixes #861

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