Bluetooth: cmtp: cmtp_add_connection() should verify that it's dealing with l2cap socket

... rather than relying on ciptool(8) never passing it anything else.  Give
it e.g. an AF_UNIX connected socket (from socketpair(2)) and it'll oops,
trying to evaluate &l2cap_pi(sock->sk)->chan->dst...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>