l2tp: correctly handle ancillary data in the ip6 recv path

l2tp_ip6 is incorrectly using the IPv4-specific ip_cmsg_recv to handle
ancillary data.  This means that socket options such as IPV6_RECVPKTINFO are
not honoured in userspace.

Convert l2tp_ip6 to use the IPv6-specific handler.

Ref: net/ipv6/udp.c

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