Fix: UST comm protocol: event notifier command is too large

Fix: UST comm protocol: event notifier command is too large

The event notifier command is larger than the current largest command,
and we don't want to break the protocol between UST and tools
needlessly.

Therefore, pass the struct lttng_ust_event_notifier _after_ struct
ustcomm_ust_msg.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ief172eaddd113ec9092dd57bad3d73fd4c29fd51