net/mlx5: allow allocated mbuf with external buffer

net/mlx5: allow allocated mbuf with external buffer

In the Rx datapath the flags in the newly allocated mbufs
are all explicitly cleared but the EXT_ATTACHED_MBUF must be
preserved. It would allow to use mbuf pools with pre-attached
external data buffers.

The vectorized rx_burst routines are updated in order to
inherit the EXT_ATTACHED_MBUF from mbuf pool private
RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF flag.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>