tun/tap: Bad packets sent to kernel via tun/tap interface

tun/tap: Bad packets sent to kernel via tun/tap interface

It was observed that under heavy traffic, VPP accidentally sent traffic
with the wrong source and destination to the tun/tap interface. Traffic
appears to be sent to the wrong direction. This problem is only
seen when worker thread is configured.

When worker thread is used, TX and RX may reside in different
core. Yet both TX and RX threads are sharing the same global variable,
namely iovecs without any mutex or memory barrier protection.
This creates a race condition when heavy traffic is blasted to VPP,
like 1000 pps.

We could create a mutex or memory barrier to ensure atomic memory access.
But why bother? It is a lot cheaper to just decouple the iovecs such
that TX and RX have their own iovecs.

Change-Id: I86a5a19bd8de54d54f32e1f0845bae6a81bbf686
Signed-off-by: Steven <sluong@cisco.com>
(cherry picked from commit 4ff586d1c6fc5c40e1548cd6f221a8a7f3ad033b)