Fix: lttng-sessiond: cpu hotplug stream number mismatch

The counter should be always increasing (kept in the channel), rather
than local to the function. This causes cpu hotplug handling to
disregard further streams that should be added to the consumer output
on CPU hotplug.

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