[PPP] L2TP: Fix skb handling in pppol2tp_recv_core

The function pppol2tp_recv_core doesn't handle non-linear packets properly.
It also fails to check the remote offset field.

This patch fixes these problems.  It also removes an unnecessary check on
the UDP header which has already been performed by the UDP layer.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>