vrf: fix maximum MTU

My initial goal was to fix the default MTU, which is set to 65536, ie above
the maximum defined in the driver: 65535 (ETH_MAX_MTU).

In fact, it's seems more consistent, wrt min_mtu, to set the max_mtu to
IP6_MAX_MTU (65535 + sizeof(struct ipv6hdr)) and use it by default.

Let's also, for consistency, set the mtu in vrf_setup(). This function
calls ether_setup(), which set the mtu to 1500. Thus, the whole mtu config
is done in the same function.

Before the patch:
$ ip link add blue type vrf table 1234
$ ip link list blue
9: blue: <NOARP,MASTER> mtu 65536 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether fa:f5:27:70:24:2a brd ff:ff:ff:ff:ff:ff
$ ip link set dev blue mtu 65535
$ ip link set dev blue mtu 65536
Error: mtu greater than device maximum.

Fixes: 5055376a

 ("net: vrf: Fix ping failed when vrf mtu is set to 0")
CC: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>