net: l2tp: ppp: change PPPOL2TP_MSG_* => L2TP_MSG_*

commit fba40c632c6473fa89660e870a6042c0fe733f8c upstream.

Signed-off-by: Asbjoern Sloth Toennesen <asbjorn@asbjorn.st>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>