app/testpmd: conserve offload flags of mbuf

app/testpmd: conserve offload flags of mbuf

This patch is to accommodate an experimental feature of mbuf - external
buffer attachment. If mbuf is attached to an external buffer, its ol_flags
will have EXT_ATTACHED_MBUF set. Without enabling/using the feature,
everything remains same.

If PMD delivers Rx packets with non-direct mbuf, ol_flags should not be
overwritten. For mlx5 PMD, if Multi-Packet RQ is enabled, Rx packets could
be carried with externally attached mbufs.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>