net: l2tp_eth: fix kernel panic on rmmod l2tp_eth

[ Upstream commit a06998b88b1651c5f71c0e35f528bf2057188ead ]

We must prevent module unloading if some devices are still attached to
l2tp_eth driver.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Tested-by: Denys Fedoryshchenko <denys@visp.net.lb>
Cc: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>