vrf: don't run conntrack on vrf with !dflt qdisc

After the below patch, the conntrack attached to skb is set to "notrack" in
the context of vrf device, for locally generated packets.
But this is true only when the default qdisc is set to the vrf device. When
changing the qdisc, notrack is not set anymore.
In fact, there is a shortcut in the vrf driver, when the default qdisc is
set, see commit dcdd43c4 ("net: vrf: performance improvements for
IPv4") for more details.

This patch ensures that the behavior is always the same, whatever the qdisc
is.

To demonstrate the difference, a new test is added in conntrack_vrf.sh.

Fixes: 8c9c296a

 ("vrf: run conntrack only in context of lower/physdev for locally generated packets")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>