net/l2tp: don't fall back on UDP [get|set]sockopt

The l2tp [get|set]sockopt() code has fallen back to the UDP functions
for socket option levels != SOL_PPPOL2TP since day one, but that has
never actually worked, since the l2tp socket isn't an inet socket.

As David Miller points out:

  "If we wanted this to work, it'd have to look up the tunnel and then
   use tunnel->sk, but I wonder how useful that would be"

Since this can never have worked so nobody could possibly have depended
on that functionality, just remove the broken code and return -EINVAL.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: James Chapman <jchapman@katalix.com>
Acked-by: David Miller <davem@davemloft.net>
Cc: Phil Turnbull <phil.turnbull@oracle.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static void pppol2tp_seq_tunnel_show(struct seq_file *m, void *v)
{
	struct l2tp_tunnel *tunnel = v;

	seq_printf(m, "\nTUNNEL '%s', %c %d\n",
		   tunnel->name,
		   (tunnel == tunnel->sock->sk_user_data) ? 'Y' : 'N',
		   atomic_read(&tunnel->ref_count) - 1);
	seq_printf(m, " %08x %ld/%ld/%ld %ld/%ld/%ld\n",
		   tunnel->debug,
		   atomic_long_read(&tunnel->stats.tx_packets),
		   atomic_long_read(&tunnel->stats.tx_bytes),
		   atomic_long_read(&tunnel->stats.tx_errors),
		   atomic_long_read(&tunnel->stats.rx_packets),
		   atomic_long_read(&tunnel->stats.rx_bytes),
		   atomic_long_read(&tunnel->stats.rx_errors));
}

static void pppol2tp_seq_tunnel_show(struct seq_file *m, void *v)
{
	struct l2tp_tunnel *tunnel = v;

	seq_printf(m, "\nTUNNEL '%s', %c %d\n",
		   tunnel->name,
		   (tunnel == tunnel->sock->sk_user_data) ? 'Y' : 'N',
		   atomic_read(&tunnel->ref_count) - 1);
	seq_printf(m, " %08x %ld/%ld/%ld %ld/%ld/%ld\n",
		   tunnel->debug,
		   atomic_long_read(&tunnel->stats.tx_packets),
		   atomic_long_read(&tunnel->stats.tx_bytes),
		   atomic_long_read(&tunnel->stats.tx_errors),
		   atomic_long_read(&tunnel->stats.rx_packets),
		   atomic_long_read(&tunnel->stats.rx_bytes),
		   atomic_long_read(&tunnel->stats.rx_errors));
}
