l2tp: don't use tunnel socket sk_user_data in ppp procfs output

l2tp's ppp procfs output can be used to show internal state of
pppol2tp. It includes a 'user-data-ok' field, which is derived from
the tunnel socket's sk_user_data being non-NULL. Use tunnel->sock
being non-NULL to indicate this instead.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>