vti6: Fix dev->max_mtu setting

vti6: Fix dev->max_mtu setting

[ Upstream commit f8a554b4aa9686bb2c12f6bae516e58783289a03 ]

We shouldn't allow a tunnel to have IP_MAX_MTU as MTU, because
another IPv6 header is going on top of our packets. Without this
patch, we might end up building packets bigger than IP_MAX_MTU.

Fixes: b96f9afee4eb ("ipv4/6: use core net MTU range checking")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>