ipv4/6: use core net MTU range checking

ipv4/6: use core net MTU range checking

ipv4/ip_tunnel:
- min_mtu = 68, max_mtu = 0xFFF8 - dev->hard_header_len - t_hlen
- preserve all ndo_change_mtu checks for now to prevent regressions

ipv6/ip6_tunnel:
- min_mtu = 68, max_mtu = 0xFFF8 - dev->hard_header_len
- preserve all ndo_change_mtu checks for now to prevent regressions

ipv6/ip6_vti:
- min_mtu = 1280, max_mtu = 65535
- remove redundant vti6_change_mtu

ipv6/sit:
- min_mtu = 1280, max_mtu = 0xFFF8 - t_hlen
- remove redundant ipip6_tunnel_change_mtu

CC: [email protected]
CC: "David S. Miller" <[email protected]>
CC: Alexey Kuznetsov <[email protected]>
CC: James Morris <[email protected]>
CC: Hideaki YOSHIFUJI <[email protected]>
CC: Patrick McHardy <[email protected]>
Signed-off-by: Jarod Wilson <[email protected]>
Signed-off-by: David S. Miller <[email protected]>