utils: ll_addr: Handle ARPHRD_IP6GRE in ll_addr_n2a()

utils: ll_addr: Handle ARPHRD_IP6GRE in ll_addr_n2a()

ll_addr_n2a() correctly prints tunnel endpoints for gre, ipip, sit
and ip6tnl, but not for ip6gre. Fix this by adding ARPHRD_IP6GRE to
IPv6 tunnel endpoing address conversion.

Before:
-------

$ ip link show
...
18: ip6tnl0: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group default
    link/tunnel6 :: brd ::
19: ip6gre0: <NOARP> mtu 1456 qdisc noop state DOWN mode DEFAULT group default
    link/gre6 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 brd \
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00

After:
------

$ ip link show
...
18: ip6tnl0: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group default
    link/tunnel6 :: brd ::
19: ip6gre0: <NOARP> mtu 1456 qdisc noop state DOWN mode DEFAULT group default
    link/gre6 :: brd ::

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>