app/testpmd: reset headroom after txonly packet allocation

app/testpmd: reset headroom after txonly packet allocation

This patch fixes txonly raw packets allocations by resetting the
available headroom.

Indeed, some PMDs such as Virtio might prepend some data to the
packet, resulting in mbuf's data_off field to be decremented each
time the mbuf gets re-allocated.

For Virtio PMD, it means that we use only single descriptors for the
first times mbufs get allocated, as at some point there is not
enough headroom to store the header.

Other alternative would be use standard API to allocate the packets,
which does reset the headroom, but the impact on performance is too
big to consider this an option.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>