can: j1939: make sure socket is held as long as session exists

We link the socket to the session to be able provide socket specific
notifications. For example messages over error queue.

We need to keep the socket held, while we have a reference to it.

Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>