vti4: Don't override MTU passed on link creation via IFLA_MTU

Don't hardcode a MTU value on vti tunnel initialization,
ip_tunnel_newlink() is able to deal with this already. See also
commit ffc2b6ee ("ip_gre: fix IFLA_MTU ignored on NEWLINK").

Fixes: 1181412c ("net/ipv4: VTI support new module for ip_vti.")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>