vrf: fix a comment about loopback device

This is a leftover of the below commit.

Fixes: 4f04256c ("net: vrf: Drop local rtable and rt6_info")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>