net: l2tp: export debug flags to UAPI

commit 41c43fbee68f4f9a2a9675d83bca91c77862d7f0 upstream.

Move the L2TP_MSG_* definitions to UAPI, as it is part of
the netlink API.

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>