app/testpmd: change rxfreet default to 32

app/testpmd: change rxfreet default to 32

To improve performance by using bulk alloc or vectored RX routines, we
need to set rx free threshold (rxfreet) value to 32, so make this the
testpmd default.

Thirty-two is the minimum setting needed to enable either the
bulk alloc or vector RX routines inside the ixgbe driver, so it's
best made the default for that reason. Please see
"check_rx_burst_bulk_alloc_preconditions()" in ixgbe_rxtx.c, and
RX function assignment logic in "ixgbe_dev_rx_queue_setup()" in
the same file.

The difference in IO performance for testpmd when called without any
optional parameters, and using 10G NICs using the ixgbe driver, can be
significant - approx 25% or more.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>