l2tp_core hooks UDP's .destroy handler to gain advance warning of a tunnel
socket being closed from userspace.  We need to do the same thing for
IP-encapsulation sockets.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>