can: fix missing decrement of j1939_proto.inuse_idx

can: fix missing decrement of j1939_proto.inuse_idx

Like other protocols on top of AF_CAN family, also j1939_proto.inuse_idx
needs to be decremented on socket dismantle.

Fixes: 6bffe88452db ("can: add protocol counter for AF_CAN sockets")
Reported-by: Oliver Hartkopp <socketcan@hartkopp.net>
Closes: https://lore.kernel.org/linux-can/7e35b13f-bbc4-491e-9081-fb939e1b8df0@hartkopp.net/
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/09ce71f281b9e27d1e3d1104430bf3fceb8c7321.1742292636.git.dcaratti@redhat.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>