Fix: sessiond: trace-kernel: function return code overwritten

Fix: sessiond: trace-kernel: function return code overwritten

A string copy, performed using `lttng_strncpy` is performed at the end
of trace_kernel_init_event_notifier_from_event_rule() and its return
value is stored in the function's `ret` variable, which is of type `enum
lttng_error_code`.

Since LTTNG_OK is not == 0, the return value is interpreted as an error,
which causes the registration of kernel-domain on-event-hit triggers
to fail.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia0bf6eb3d2a4fca8f9089ecd4fef7a311354560c