net: vxlan: properly cleanup devs on module unload

We should use vxlan_dellink() handler in vxlan_exit_net(), since
i) we're not in fast-path and we should be consistent in dismantle
just as we would remove a device through rtnl ops, and more
importantly, ii) in case future code will kfree() memory in
vxlan_dellink(), we would leak it right here unnoticed. Therefore,
do not only half of the cleanup work, but make it properly.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>