Fix: event notifier: racy use of last subbuffer record

Fix: event notifier: racy use of last subbuffer record

The lttng-modules event notifiers use the ring buffer internally. When
reading the payload of the last event in a sub-buffer with a multi-part
read (e.g. two read system calls), we should not "put" the sub-buffer
holding this data, else continuing reading the data in the following
read system call can observe corrupted data if it has been concurrently
overwritten by the producer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Idb051e50ee8a25958cfd63a9b143f4943ca2e01a