l2tp: show IPv6 addresses in l2tp debugfs file

If an L2TP tunnel uses IPv6, make sure the l2tp debugfs file shows the
IPv6 address correctly.

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