Fix: missing session reference release on kernel poll update

The iteration performed on all sessions in update_kernel_poll() does
not release the reference taken on the sessions. This causes the
session(s) to be leaked and prevents the shutdown of the session
daemon as it waits for all sessions to be destroyed before completing
its teardown.

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