vti6: Change minimum MTU to IPV4_MIN_MTU, vti6 can carry IPv4 too

A vti6 interface can carry IPv4 as well, so it makes no sense to
enforce a minimum MTU of IPV6_MIN_MTU.

If the user sets an MTU below IPV6_MIN_MTU, IPv6 will be
disabled on the interface, courtesy of addrconf_notify().

Reported-by: Xin Long <lucien.xin@gmail.com>
Fixes: b96f9afe ("ipv4/6: use core net MTU range checking")
Fixes: c6741fbe ("vti6: Properly adjust vti6 MTU from MTU of lower device")
Fixes: 53c81e95

 ("ip6_vti: adjust vti mtu according to mtu of lower device")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>