netdev-dpdk: Fix deadlock in destroy_device().

netdev-dpdk: Fix deadlock in destroy_device().

netdev_dpdk_vhost_destruct() calls rte_vhost_driver_unregister(), which
can trigger the destroy_device() callback.  destroy_device() will try to
take two mutexes already held by netdev_dpdk_vhost_destruct(), causing a
deadlock.

This problem can be solved by dropping the mutexes before calling
rte_vhost_driver_unregister().  The netdev_dpdk_vhost_destruct() and
construct() call are already serialized by netdev_mutex.

This commit also makes clear that dev->vhost_id is constant and can be
accessed without taking any mutexes in the lifetime of the devices.

Fixes: 8d38823bdf8b("netdev-dpdk: fix memory leak")
Reported-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Mark Kavanagh <mark.b.kavanagh@intel.com>