l2tp: cleanup netlink send of tunnel address information

l2tp_nl_tunnel_send has conditionally compiled code to support AF_INET6,
which makes the code difficult to follow and triggers checkpatch
warnings.

Split the code out into functions to handle the AF_INET v.s. AF_INET6
cases, which both improves readability and resolves the checkpatch
warnings.

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