net: vrf: set operstate and mtu at link create

The VRF device exists to define L3 domains and guide FIB lookups. As
such its operstate is not relevant. Seeing 'state UNKNOWN' in the
output of 'ip link show' can be confusing, so set operstate at link
create.

Similarly, the MTU for a VRF device is not used; any fragmentation
of the payload is done on the output path based on the real egress
device. An MTU of 1500 on the VRF device while enslaved devices
have a higher MTU can lead to confusion. Since the VRF MTU is not
relevant set to 64k similar to what is done for loopback.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>