can: j1939: socket: rework socket locking for j1939_sk_release() and j1939_sk_sendmsg()

j1939_sk_sendmsg() should be protected by lock_sock() to avoid race with
j1939_sk_bind() and j1939_sk_release().

Reported-by:  <syzbot+afd421337a736d6c1ee6@syzkaller.appspotmail.com>
Reported-by:  <syzbot+6d04f6a1b31a0ae12ca9@syzkaller.appspotmail.com>
Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>