can: j1939: move j1939_priv_put() into sk_destruct callback

This patch delays the j1939_priv_put() until the socket is destroyed via
the sk_destruct callback, to avoid use-after-free problems.

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