BugLink: https://bugs.launchpad.net/bugs/1775771

[ Upstream commit 2b3957c3 ]

Commit 128bb975 ("ip6_gre: init dev->mtu and dev->hard_header_len
correctly") fixed IFLA_MTU ignored on NEWLINK for ip6_gre. The same
mtu fix is also needed for sit.

Note that dev->hard_header_len setting for sit works fine, no need to
fix it. sit is actually ipv4 tunnel, it can't call ip6_tnl_change_mtu
to set mtu.
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>