can: af_can: export can_sock_destruct()

In j1939 we need our own struct sock::sk_destruct callback. Export the
generic af_can can_sock_destruct() that allows us to chain-call it.

Fixes: 9d71dd0c

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