net/ena: fix releasing Tx ring mbufs

net/ena: fix releasing Tx ring mbufs

When ena_tx_queue_release_bufs() frees the mbufs it does not clear
the mbuf pointers.  So, when the device starts and stops multiple
times it can cause the application to receive duplicate mbufs for
two different packets.  Fix the issue by clearing the mbuf pointer.

Also, while tracking down the "double free" issue the ena calls to
allocate and free mbufs in bulk were migrated to the mbuf based APIs
so the common mbuf alloc/free routines are exercised.

Fixes: 79405ee17585 ("net/ena: fix out of order completion")
Fixes: 1173fca25af9 ("ena: add polling-mode driver")
Cc: stable@dpdk.org
Signed-off-by: David Harton <dharton@cisco.com>
Acked-by: Michal Krawczyk <mk@semihalf.com>