vhost-user: restart vpp may cause vhost to crash
vhost-user: restart vpp may cause vhost to crash

Fix a typo in vhost_user_rx_discard_packet which may cause
txvq->last_avail_idx to go wild.

Change-Id: Ifaeb58835dff9b7ea82c061442722f1dcaa5d9a4
Signed-off-by: Steven Luong <a748ea44ad7b179c2313a856d44e1e4eb39869bb@cisco.com>
(cherry picked from commit 39382976701926c1f34191c1311829c15a53cb01)

chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp