app/testpmd: fix forward ports Rx flush

app/testpmd: fix forward ports Rx flush

A port Rx queue flush is done when the packet forwarding starts in
order to clean the port statistics for a new traffic session.

The flush operation is wrongly called before the update of the new
forward ports, and may fail due to flush operation for an invalid port
configured by the old session.

Move the new forward port setup to be done before the Rx queue flush.

Fixes: 7741e4cf16c0 ("app/testpmd: VMDq and DCB updates")
Cc: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>