l2tp: don't use sk_user_data in l2tp_udp_encap_err_recv

If UDP sockets are aliased, sk might be the wrong socket. There's no
benefit to using sk_user_data to do some checks on the associated
tunnel context. Just report the error anyway, like udp core does.

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