tap: crash in multi-thread environment

tap: crash in multi-thread environment

In tap tx routine, virtio_interface_tx_inline, there used to be an
interface spinlock to ensure packets are processed in an orderly fashion
  clib_spinlock_lock_if_init (&vif->lockp);

When virtio code was introduced in 19.04, that line is changed to
  clib_spinlock_lock_if_init (&vring->lockp);
to accommodate multi-queues.

Unfortunately, althrough the spinlock exists in the vring, it was never
initialized for tap, only for virtio. As a result, many nasty things can
happen when running tap interface in multi-thread environment. Crash is
inevitable.

The fix is to initialize vring->lockp for tap and remove vif->lockp as it
is not used anymore.

Change-Id: Ibc8f5c8192af550e3940597c06992dfdaccb4c49
Signed-off-by: Steven Luong <sluong@cisco.com>