Fix: sessiond: silent kernel notifier registration error

Fix: sessiond: silent kernel notifier registration error

When a kernel notifier fails to be registered, the error is silently
ignored because of a missing jump to the error label.

This was noticed while debugging an unrelated problem where the kernel
tracer would fail to add a userspace probe to a semaphore-protected SDT
probe.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9261ad7131b1b53152fe8960f17a6fca7029440f