Merge branch 'tunnel-mtus'

Nicolas Dichtel says:

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ip[6] tunnels: fix mtu calculations

The first patch restores the possibility to bind an ip4 tunnel to an
interface whith a large mtu.
The second patch was spotted after the first fix. I also target it to net
because it fixes the max mtu value that can be used for ipv6 tunnels.

v2: remove the 0xfff8 in ip_tunnel_newlink()
====================

Signed-off-by: David S. Miller <davem@davemloft.net>