netdev-dpdk: Restrict vhost_sock_dir

netdev-dpdk: Restrict vhost_sock_dir

Since the vhost-user sockets directory now comes from the database, it is
possible for any user with database access to program an arbitrary filesystem
location for the sockets directory. This could result in unprivileged users
creating or deleting arbitrary filesystem files by using specially crafted
names. To prevent this, 'vhost-sock-dir' is now relative to ovs_rundir()
and must not contain "..".

Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>