Fix: use after free in ring buffer clients

Don't use ring buffer client's struct lttng_channel from ioctl which
applies to ring buffer streams, because lttng_channel is freed while lib
ring buffer stream and channel are still in use. Their lifetime persists
until the consumer daemon releases its handles on the related stream
file descriptors.

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