ip6_gre: Fix MTU setting

In ip6gre_tnl_link_config set t->tun_len and t->hlen correctly for the
configuration. For hard_header_len and mtu calculation include
IPv6 header and encapsulation overhead.

In ip6gre_tunnel_init_common set t->tun_len and t->hlen correctly for
the configuration. Revert to setting hard_header_len instead of
needed_headroom.

Tested:

./ip link add name tun8 type ip6gretap remote \
2401:db00:20:911a:face:0:27:0 local \
2401:db00:20:911a:face:0:25:0 ttl 225

Gives MTU of 1434. That is equal to 1500 - 40 - 14 - 4 - 8.

./ip link add name tun8 type ip6gretap remote \
2401:db00:20:911a:face:0:27:0 local \
2401:db00:20:911a:face:0:25:0 ttl 225 okey 123

Gives MTU of 1430. That is equal to 1500 - 40 - 14 - 4 - 8 - 4.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>