Fix: error handling in kernel add_channel

consumer_add_channel can return an error, it needs to be handled. Also,
init_kernel_tracing returns a positive lttng_error_code on error, so the
check in start_kernel_session needs to be for ret != 0.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>