vxlan: fix oops in dev_fill_metadata_dst

Since the commit 0c1d70af ("net: use dst_cache for vxlan device")
vxlan_fill_metadata_dst() calls vxlan_get_route() passing a NULL
dst_cache pointer, so the latter should explicitly check for
valid dst_cache ptr. Unfortunately the commit d71785ff ("net: add
dst_cache to ovs vxlan lwtunnel") removed said check.

As a result is possible to trigger a null pointer access calling
vxlan_fill_metadata_dst(), e.g. with:

ovs-vsctl add-br ovs-br0
ovs-vsctl add-port ovs-br0 vxlan0 -- set interface vxlan0 \
	type=vxlan options:remote_ip=192.168.1.1 \
	options:key=1234 options:dst_port=4789 ofport_request=10
ip address add dev ovs-br0 172.16.1.2/24
ovs-vsctl set Bridge ovs-br0 ipfix=@i -- --id=@i create IPFIX \
	targets=\"172.16.1.1:1234\" sampling=1
iperf -c 172.16.1.1 -u -l 1000 -b 10M -t 1 -p 1234

This commit addresses the issue passing to vxlan_get_route() the
dst_cache already available into the lwt info processed by
vxlan_fill_metadata_dst().

Fixes: d71785ff ("net: add dst_cache to ovs vxlan lwtunnel")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>