ipv6: gre: setup default multicast routes over PtP links

GRE point-to-point interfaces should also support ipv6 multicast. Setting
up default multicast routes on interface creation was forgotten. Add it.

Bugzilla: <https://bugzilla.kernel.org/show_bug.cgi?id=103231

>
Cc: Julien Muchembled <jm@jmuchemb.eu>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Nicolas Dumazet <ndumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>