ip: link_gre6.c: Support IP6_TNL_F_ALLOW_LOCAL_REMOTE flag

ip: link_gre6.c: Support IP6_TNL_F_ALLOW_LOCAL_REMOTE flag

For IP-in-IP tunnels, one can specify the [no]allow-localremote command
when configuring a device. Under the hood, this flips the
IP6_TNL_F_ALLOW_LOCAL_REMOTE flag on the netdevice. However, ip6gretap
and ip6erspan devices, where the flag is also relevant, are not IP-in-IP
tunnels, and thus there's no way to configure the flag on these
netdevices. Therefore introduce the command to link_gre6 as well.

The original support was introduced in commit 21440d19d957
("ip: link_ip6tnl.c/ip6tunnel.c: Support IP6_TNL_F_ALLOW_LOCAL_REMOTE flag")

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>